delivered the opinion of the Court.

This case involves the validity of an Alien Registration Act adopted by the Commonwealth of Pennsylvania.1 The Act, passed in 1939, requires every alien 18 years or over, with certain exceptions,2 to register once each year ; provide such information as is required by the statute, plus any “other information and details” that the Department of Labor and Industry may direct; pay $1 as an annual registration fee; receive an alien identification card and carry it at all times; show the card whenever it may be demanded by any police officer or any agent of the Department of Labor and Industry; and exhibit the card as a condition precedent to registering a motor vehicle in his name or obtaining a license to operate one. The Department of Labor and Industry is charged with the duties of classifying the registrations for “the purpose of ready reference,” and furnishing a copy of the classification to the Pennsylvania Motor Police. Nonexempt aliens who fail to register are subject to a fine of not *60more than $100 or' imprisonment for not more than 60 days, or both. For failure to carry an identification card or for failure to show it upon proper demand, the punishment is a fine of not more than $10, or imprisonment for not more than 10 days, or both.

A three-judge District Court enjoined enforcement of the Act, holding that it denied aliens equal protection of the laws, and that it encroached upon legislative powers constitutionally vested in the federal government.3 It is that judgment we are here called upon to review.4 But in 1940, after the court had held the Pennsylvania Act invalid, Congress enacted a federal Alien Registration Act.5 We must therefore pass upon the state Act in the light of the Congressional Act.6

The federal Act provides for a single registration of aliens 14 years of age and over; detailed information specified by the Act, plus “such additional matters as may be prescribed by the Commissioner, with the approval of the Attorney General”; finger-printing of all registrants; and secrecy of the federal files, which can be “made available only to such persons or agencies as may be designated by the Commissioner, with the approval of the Attorney General.” No requirement that aliens carry a registration card to be exhibited to police or *61others is embodied in the law, and only the wilful failure to register is made a criminal offense; punishment is fixed at a fine of not more than $1000, imprisonment for not more than 6 months, or both.

The basic subject of the state and federal laws is identical — registration of aliens as a distinct group. Appellants urge that the Pennsylvania law “was constitutional when passed,” and that “The only question is whether the state act is in abeyance or whether the state and Federal Government have concurrent jurisdiction to register aliens for the protection of inhabitants and property.” Appellees, on the other hand, contend that the Pennsylvania Act is invalid, for the reasons that it (1) denies equal protection of the laws to aliens residing in the state; (2) violates § 16 of the Civil Rights Act of 1870;7 (3) exceeds Pennsylvania’s constitutional power in requiring registration of aliens without Congressional consent. Appellees’ final contention is that the power to restrict, limit, regulate and registefeahj*ijg& a distinct group is not an equal and continucmiH^BHfeg concurrent power of state and nation, but thafillpn if the state can legislate on this subject at all, its power is subordinate to supreme national law. Appellees conclude that by its adoption of a comprehensive, integrated scheme for regulation of aliens — including its 1940 registration act — Congress has precluded state action like that taken by Pennsylvania.8

*62In the view we take it is not necessary to pass upon appellees’ first, second, and third contentions, and so we pass immediately to their final question, expressly leaving open all of appellees’ other contentions, including the argument that the federal power in this field, whether exercised or unexercised, is exclusive. Obviously the answer to appellees’ final question depends upon an analysis of the respective powers of state and national governments in the regulation of aliens as such, and a determination of whether Congress has, by its action, foreclosed enforcement of Pennsylvania’s registration law.

First. That the supremacy of the national power in the general field of foreign affairs, including power over immigration, naturalization and deportation, is made clear by the Constitution, was pointed out by the authors of The Federalist in 1787,9 and has since been given continuous recognition by this Court.10 When the national governmenJáMÉamty or statute has established rules and *63regulations touching the rights, privileges, obligations or burdens of aliens as such, the treaty or statute is the supreme law of the land. No state can add to or take from the force and effect of such treaty or statute, for Article VI of the Constitution provides that “This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land; and the Judges in every State shall be bound thereby, any Thing in the Constitution or Laws of any State to the Contrary notwithstanding.” The Federal Government, representing as it does the collective interests of the forty-eight states, is entrusted with full and exclusive responsibility for the conduct of affairs with foreign sovereignties. “For local interests the several States of the Union exist, but for national purposes, embracing our relations with foreign nations, we are but one people, one nation, one power.”11 Our system of government is such that the interest of the cities, counties and states, no less than the interest of the people of the whole nation, imperatively requires that federal power in the field affecting foreign relations be left entirely free from local interference. As Mr. Justice Miller well observed of a California *64statute burdening immigration: “If [the United States] should get into a difficulty which would lead to war, or to suspension of intercourse, would California alone suffer, or all the Union?”12

One of the most important and delicate of all international relationships, recognized immemorially as a responsibility of government, has to do with the protection of the just rights of a country’s own nationals when those nationals are in another country. Experience has shown that international controversies of the gravest moment, sometimes even leading to war, may arise from real or imagined wrongs to another’s subjects inflicted, or permitted, by a government.13 This country, like other nations, has entered into numerous treaties of amity and commerce since its inception — treaties entered into under express constitutional authority, and binding *65upon the states as well as the nation. Among those treaties have been many which not only promised and guaranteed broad rights and privileges to aliens sojourning in our own territory, but secured reciprocal promises and guarantees for our own citizens while in other lands. And apart from treaty obligations, there has grown up in the field of international relations a body of customs defining with more or less certainty the duties owing by all nations to alien residents — duties which our State Department has often successfully insisted foreign nations must recognize as to our nationals abroad.14 In general, both treaties and international practices have been aimed at preventing injurious discriminations against aliens. Concerning such treaties, this Court has said: “While treaties, in safeguarding important rights in the interest of reciprocal beneficial relations, may by their express terms afford a measure of protection to aliens which citizens of one or both of the parties may not be able to demand against their own government, the general purpose of treaties of amity and commerce is to avoid injurious discrimination in either country against the citizens of the other.” 15

Legal imposition of distinct, unusual and extraordinary burdens and obligations upon aliens — such as subjecting *66them alone, though perfectly law-abiding, to indiscriminate and repeated interception and interrogation by public officials — thus bears an inseparable relationship to the welfare and tranquillity of all the states, and not merely to the welfare and tranquillity of one. Laws imposing such burdens are not mere census requirements, and even though they may be immediately associated with the accomplishment of a local purpose, they provoke questions in the field of international affairs. And specialized regulation of the conduct of an alien before naturalization is a matter which Congress must consider in discharging its constitutional duty “To establish an Uniform Rule of Naturalization . . .” It cannot be doubted that both the state and the federal registration laws belong “to that class of laws which concern the exterior relation of this whole nation with other nations and governments.”16 Consequently the regulation of aliens is so intimately blended and intertwined with responsibilities of the national government that where it acts, and the state also acts on the same subject, “the act of Congress, or the treaty, is supreme; and the law of the State, though enacted in the exercise of powers not controverted, must yield to it.” 17 And where the federal government, in the exercise of its superior authority in this field, has enacted a complete scheme of regulation and has therein provided a standard for the registration of aliens, states cannot, inconsistently with the purpose of Congress, conflict or interfere with, curtail or complement, the federal law, or enforce additional or aux*67iliary regulations.18 There is not — and from the very nature of the problem there cannot be — any rigid formula or rule which can be used as a universal pattern to determine the meaning and purpose of every act of Congress. This Court, in considering the validity of state laws in the light of treaties or federal laws touching the same subject, has made use of the following expressions: conflicting; contrary to; occupying the field; repugnance; difference; irreconcilability; inconsistency; violation; curtailment; and interference.19 But none of these expressions provides an infallible constitutional test or an exclusive constitutional yardstick. In the final analysis, there can be no one crystal clear distinctly marked formula. Our primary function is to determine whether, under the circumstances of this particular case, Pennsylvania’s law stands as an obstacle to the accomplishment and execution of the full purposes and objectives of Congress.20 And in *68that determination, it is of importance that this legislation is in a field which affects international relations, the one aspect of our government that from the first has been most generally conceded imperatively to demand broad national authority. Any concurrent state power that may exist is restricted to the narrowest of limits; the state’s power here is not bottomed on the same broad base as is its power to tax.21 And it is also of importance that, this legislation deals with the rights, liberties, and personal freedoms of human beings, and is in an entirely different category from state tax statutes or state pure food laws regulating the labels on cans.22

Our conclusion is that appellee is correct in his contention that the power to restrict, limit, regulate, and register aliens as a distinct group is not an equal and continuously existing concurrent power of state and nation, but that whatever power a state may have is subordinate to supreme national law.23 We proceed there*69fore to an examination of Congressional enactments to ascertain whether or not Congress has acted in such manner that its action should preclude enforcement of Pennsylvania’s law.

Second. For many years Congress has provided a broad and comprehensive plan describing the terms and conditions upon which aliens may enter this country, how they may acquire citizenship, and the manner in which they may be deported. Numerous treaties, in return for reciprocal promises from other governments, have pledged the solemn obligation of this nation to the end that aliens residing in .our territory shall not be singled out for the imposition of discriminatory burdens. Our Constitution and our Civil Rights Act have guaranteed to aliens “the equal protection of the laws [which] is a pledge of the protection of equal laws.”24 With a view to limiting prospective residents from foreign lands to those possessing the qualities deemed essential to good and useful citizenship in America, carefully defined qualifications are required to be met before aliens may enter our country. These qualifications include rigid requirements as to health, education, integrity, character, and adaptability to our institutions. Nor is the alien left free from the *70application of federal laws after entry and before naturalization. If during the time he is residing here he should be found guilty of conduct contrary to the rules and regulations laid down by Congress, he can be deported. At the time he enters the country, at the time he applies for permission to acquire the full status of citizenship, and during the intervening years, he can be subjected to searching investigations as to conduct and suitability for citizenship.25 And in 1940 Congress added to this comprehensive scheme a complete system for alien registration.

The nature of the power exerted by Congress, the object sought to be attained, and the character of the obligations imposed by the law, are all important in considering the question of whether supreme federal enactments preclude enforcement of state laws on the same subject.26 Opposition to laws permitting invasion of the personal liberties of law-abiding individuals, or singling out aliens as particularly dangerous and undesirable groups, is deep-seated in this country. Hostility to such legislation in America stems back to our colonial history,27 and champions of freedom for the individual have always vigorously opposed burdensome registration systems. The drastic requirements of the alien Acts of 179828 brought about a political upheaval in this country the repercussions from which have not even yet wholly subsided.29 So violent was the reaction to the 1798 laws that almost a century elapsed before a second registration *71act was passed. This second law, which required Chinese to register and carry identification cards with them at all times, was enacted May 5, 1892. An opponent of this legislation, speaking in the Senate of the requirement that cards be carried, said: “[The Chinese covered by the Act] are here ticket-of-leave men; precisely as, under the Australian law, a convict is allowed to go at large upon a ticket-of-leave, these people are to be allowed to go at large and earn their livelihood, but they must have their tickets-of-leave in their possession. . . . This inaugurates in our system of government a new departure; one, I believe never before practised, although it was suggested in conference that some such rules had been adopted in slavery times to secure the peace of society.”30

For many years bills have been regularly presented to every Congress providing for registration of aliens. Some of these bills proposed annual registration of aliens, issuance of identification cards containing information about and a photograph of the bearer, exhibition of the cards on demand, payment of an annual fee, and kindred requirements.31 Opposition to these bills was based upon charges that their requirements were at war with the fundamental principles of our free government, in that they would bring about unnecessary and irritating restrictions upon personal liberties of the individual, and would subject aliens to a system of indiscriminate questioning similar to the espionage systems existing in other lands.32

*72When Congress passed the Alien Registration Act of 1940, many of the provisions which had been so severely criticized were not included.33 The Congressional purpose, as announced by the chairman of the Senate subcommittee which drafted the final bill, was to “work . . . the new provisions into the existing [immigration and naturalization] laws, so as to make a harmonious whole.” 34 That “harmonious whole” included the “Uniform Rule of Naturalization” the Constitution empow*73ered the Congress to provide.35 And as a part of that “harmonious whole,” under the federal Act aliens need not carry cards, and can only be punished for wilful failure to register.36 Further, registration records and fingerprints must be kept secret and cannot be revealed except to agencies — such as a state — upon consent of the Commissioner and the Attorney General.

We have already adverted to the conditions which make the treatment of aliens, in whatever state they may be located, a matter of national moment. And whether or not registration of aliens is of such a nature that the Constitution permits only of one uniform national system, it cannot be denied that the Congress might validly conclude that such uniformity is desirable. The legislative history of the Act indicates that Congress was trying to steer a middle path, realizing that any registration requirement was a departure from our traditional policy of not treating aliens as a thing apart, but also feeling that the Nation was in need of the type of information to *74be secured.37 Having the constitutional authority so to do, it has provided a standard for alien registration in a single integrated and all-embracing system in order to obtain the information deemed to be desirable in connection with aliens. When it made this addition to its uniform naturalization and immigration laws, it plainly manifested a purpose to do so in such a way as to protect the personal liberties of law-abiding aliens through one uniform national registration system, and to leave them free from the possibility of inquisitorial practices and police surveillance that might not only affect our international relations but might also generate the very disloyalty which the law has intended guarding against. Under these circumstances, the Pennsylvania Act cannot be enforced. Accordingly, the judgment below is

Affirmed.

dissenting:

I think the judgment below should be reversed.

Undoubtedly Congress, in the exercise of its power to legislate in aid of powers granted by the Constitution to the national government may greatly enlarge the exercise of federal authority and to an extent which need not now be defined, it may, if such is its will, thus subtract from the powers which might otherwise be exercised by *75the states. Assuming, as the Court holds, that Congress could constitutionally set up an exclusive registration system for aliens, I think it has not done so and that it is not the province of the courts to do that which Congress has failed to do.

At a time when the exercise of the federal power is being rapidly expanded through Congressional action, it is difficult to overstate the importance of safeguarding against such diminution of state power by vague inferences as to what Congress might have intended if it had considered the matter or by reference to our own conceptions of a policy which Congress has not expressed and which is not plainly to be inferred from the legislation which it has enacted. Cf. Graves v. O’Keefe, 306 U. S. 466, 479, 480, 487. The Judiciary of the United States should not assume to strike down a state law which is immediately concerned with the social order and safety of its people unless the statute plainly and palpably violates some right granted or secured to the national government by the Constitution or similarly encroaches upon the exercise of some authority delegated to the United States for the attainment of objects of national concern.

The opinion of the Court does not deny, and I see no reason to doubt that the Pennsylvania registration statute, when passed, was a lawful exercise of the constitutional power of the state. With exceptions not now material it requires aliens resident in the state, who have not declared their intention to become citizens, to register annually, to pay a registration fee of $1.00, and to carry a registration identification card. It affords to the state a convenient method of ascertaining the number and whereabouts of aliens within the state, which it is entitled to know, and a means of their identification. It is an available aid in the enforcement of a number of statutes of the state applicable to aliens whose constitu*76tional validity has not been questioned, one of which has been held by this Court not to infringe the Fourteenth Amendment. Patsone v. Pennsylvania, 232 U. S. 138.1

The national government has exclusive control over the admission of aliens into the United States but, after entry, an alien resident within a state, like a citizen, is subject to the police powers of the state and, in the exercise of that power, state legislatures may pass laws applicable exclusively to aliens so long as the distinction taken between aliens and citizens is not shown to be without rational basis. Patsone v. Pennsylvania, supra; Terrace v. Thompson, 263 U. S. 197; Cockrill v. California, 268 U. S. 258; Ohio v. Deckebach, 274 U. S. 392, 396, and cases cited. The federal government has no general police power over aliens and, so far as it can exercise any control over them, it must be in the pursuance of a power granted to it by the Constitution.

The opinion of the Court does not support its conclusion upon the ground that in the absence of federal legislation on the subject there is any want of power in the state to pass the present statute. It does not suggest, nor could it well do so, that in the absence of Congressional action the Pennsylvania statute either by its own terms or in its operation interferes with or obstructs the author*77ity conferred by the Constitution on the national government over the national defense, the conduct of foreign relations, its powers over immigration and deportation of aliens or their naturalization. The existence of the national power to conduct foreign relations and negotiate treaties does not foreclose state legislation dealing exclusively with aliens as such. This. Court has consistently held that treaties of the United States for the protection of resident aliens do not supersede such legislation unless they conflict with it. See Ohio v. Deckebach, supra, 395 and cases cited; Todok v. Union State Bank, 281 U. S. 449, 454 et seq.; cf. Nielsen v. Johnson, 279 U. S. 47. It is not contended that the Pennsylvania statute conflicts with any term of any treaty.

The question presented here is a different one from that considered in Henderson v. Mayor of New York, 92 U. S. 259, 273, where the state taxation and registration of all persons entering the United States through a port of the state was held to be a regulation of foreign commerce forbidden to the states by the Constitution, even though Congress had passed no similar legislation. The registration of aliens resident in a state is not a regulation of interstate or foreign commerce or of the entry or deportation of aliens and would seem to be no more an exercise of any power granted to the national government, or an encroachment upon it, than is a state census for local purposes an infringement of the national authority to take a national census for national purposes. It is the federal act alone which is pointed to as curtailing or withdrawing the reserved power of the state over its alien population.

Title I of the federal statute penalizes certain acts of any persons intended to interfere with, impair or influence the loyalty, morale or discipline of the military or naval forces of the United States. Title II, among other things, provides for the deportation of aliens after con*78viction and service of sentence for violations of the provisions of Title I. And the evident purpose of the registration provisions of Title III is to aid in the enforcement of the other provisions of the Act and in the prevention of subversive activities! of aliens resident within the United States. It requires the registration and fingerprinting of all aliens over fourteen years of age, with exceptions not now material, who are not registered and fingerprinted upon entering the country. Registered aliens resident in the United States are required to notify the Commissioner of Immigration of any change of residence and penalties are imposed for wilful non-compliance. As construed and applied by the opinion of the Court the federal act denies to the states the practicable means of identifying their alien residents and of recording their whereabouts and it withholds from the states the benefit of the information secured under the federal act except insofar as it may be made available to them on application to the Attorney General.

It is conceded that the federal act in operation does not at any point conflict with the state statute, and it does not by its terms purport to control or restrict state authority in any particular. But the government says that Congress by passing the federal act, has “occupied the field” so as to preclude the enforcement of the state statute and that the administration of the latter might well conflict with Congressional policy to protect the civil liberty of aliens against the harassments of intrusive police surveillance.

Little aid can be derived from the vague and illusory but often repeated formula that Congress “by occupying the field” has excluded from it all state legislation. Every Act of Congress occupies some field, but we must know the boundaries of that field before we can say that it has precluded a state from the exercise of any power reserved to it by the Constitution. To discover *79the boundaries we look to the federal statute itself, read in the light of its constitutional setting and its legislative history.

Federal statutes passed in aid of a granted power obviously supersede state statutes with which they conflict. Pennsylvania R. Co. v. Illinois Brick Co., 297 U. S. 447, 459. See Kelly v. Washington, 302 U. S. 1, 10. But we are pointed to no such conflict here. In the exercise of such powers Congress also has wide scope for prohibiting state regulation of matters which Congress may, but has not undertaken to regulate itself. But no words of the statute or of any comihittee report, or any Congressional debate indicate that Congress intended to withdraw from the states any part of their constitutional power over aliens within their borders. We must take it that Congress was not unaware that some nineteen states have statutes or ordinances requiring some form of registration for aliens, seven of them dating from the last war. The repeal of this legislation is not to be inferred from the silence of Congress in enacting a law which at no point conflicts with the state legislation and is harmonious with it.

The exercise of the federal legislative power is certainly not more potent to curtail the exercise of state power over aliens than is the exercise of the treaty making power. Yet as we have seen no treaty has that effect unless it conflicts with a state statute. The passage of the National Pure Food & Drug Act did not preclude the states from supplementing it by like additional requirements not conflicting with those of the Congressional act. Savage v. Jones, 225 U. S. 501. The enactment of federal laws for the inspection, as a safety measure, of vessels plying navigable waters of the United States does not foreclose the states from like inspection of the hull and machinery of such vessels within the state, to insure safety and determine seaworthiness, demands *80which lie outside the federal requirements. Kelly v. Washington, supra. The passage of the National Draft and the National Espionage Acts with their penalties for violation, did not preclude a state from making it a misdemeanor for any person to advocate that citizens of the state refuse to aid or assist the United States in carrying on a war. Gilbert v. Minnesota, 254 U. S. 325; cf. Halter v. Nebraska, 204 U. S. 34; see also Reid v. Colorado, 187 U. S. 137; Carey v. South Dakota, 250 U. S. 118; Dickson v. Uhlmann Grain Co., 288 U. S. 188; Mintz v. Baldwin, 289 U. S. 346; Maurer v. Hamilton, 309 U. S. 598, 614. These are but a few of the many examples of the long established principle of constitutional interpretation that an exercise by the state of its police power, which would be valid if not superseded by federal action, is superseded only where the repugnance or conflict is so “direct and positive” that the two acts cannot “be fairly reconciled or consistently stand together.” Sinnot v. Davenport, 22 How. 227, 243; Kelly v. Washington, supra, 10. A federal registration act designed to aid in enforcing federal statutes and to prevent subversive activities against the national government can stand consistently with a like statute applicable to residents passed in aid of state laws-and as a safeguard to property and persons within the state, as readily as the federal and state laws which annually demand two separate income tax returns of the citizen.

The Fourteenth Amendment guarantees the civil liberties of aliens as well as of citizens against infringement by state action in the enactment of laws and their administration as well. Again we are pointed to nothing in the Federal Alien Registration Act or in the records of its passage through Congress to indicate that Congress thought those guarantees inadequate or that in requiring registration of all aliens it undertook to prevent the states from passing any registration measure otherwise constitutional. True, it was careful to bring the new *81legislation into harmony with existing federal statutes and to avoid, so far as consistent with its purposes, any harsh or oppressive requirements, but in all this there is to be found no warrant for saying that there was. a Congressional purpose to curtail the exercise of any constitutional power of the state over its alien residents or to protect the alien from state action which the Constitution prohibits and which the federal courts stand ready to prevent. See Hague v. C. I. O., 307 U. S. 496, 518, 525 et seq.

Here compliance with the state law does not preclude or even interfere with compliance with the act of Congress. The enforcement of both acts involves no more inconsistency, no more inconvenience to the individual, and no more embarrassment to either government than do any of the laws, state and national, such as revenue laws, licensing laws, or police regulations, where interstate commerce is involved, which are equally applied to the citizen because he is subject, as are aliens, to a dual sovereignty.