Steven Luokkala appeals the sentence he received after pleading guilty to being a felon in possession of a firearm. In a supplemental brief, 1 Luokkala challenges the validity of his sentence in light of United States v. Booker, — U.S. -, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005). Finding Luokkala’s Booker argument valid, we vacate his sentence and remand for resentencing.

Under section 4A1.2(e) of the United States Sentencing Guidelines, only prior sentences imposed within the specified time period—ten years before the “commencement of the instant offense” for any sentence not exceeding one year and one month—can be counted in determining a defendant’s criminal history category, which impacts the defendant’s ultimate sentencing range. U.S. Sentencing Guidelines Manual § 4A1.2(e) (2003). The Guidelines further instruct courts to consider “any relevant conduct” in ascertaining the commencement of the offense. Id. at. § 4A1.2, cmt. n. 8.

Luokkala’s presentence investigation report, which the district court ultimately adopted, counted sentences imposed between 1988 and 1992 based on the apparent finding that Luokkala had possessed firearms since 1996. In Luokkala’s view, this finding, which increased his criminal history category and thus lengthened his sentence, violated his Sixth Amendment rights as articulated in Booker. But the Government, urging the court to equate Luokkala’s failure to object to the facts underlying the relevant-conduct finding with an admission of the truth of those facts, insists that the finding comports with Booker. We find it unnecessary to resolve this dispute because, regardless of the merits of Luokkala’s Sixth Amendment claim, the district court’s treatment of the Guidelines as mandatory constitutes plain error under Booker and requires remand here. See United States v. Barnett, 398 F.3d 516, 530 (6th Cir. 2005).

The district court’s grant of a downward departure under section 4A1.3(b)(1) of the Guidelines on the ground that Luokkala’s criminal history category “substantially over-represents the seriousness of the defendant’s criminal history,” does not alter our view that Booker requires remand here. We find unpersuasive the Government’s contention that the district court “was not sentencing within a mandatory scenario” because the decision to de *497 part downward gave it “unfettered discretion to fashion [a] sentence.” Though the departure arguably gave the district court discretion to impose any criminal history category it deemed reasonable, the court remained constrained by the offense level.

Accordingly, we vacate Luokkala’s sentence and remand for resentencing under Booker.