Anyone can fork at any time. The US gov could theoretically hold Wikipedia's brand and servers hostage, but the actually valuable stuff is already mirrored in a decentralized fashion that is completely unrestricted under US and international law.

EDIT: Maybe you meant that the US could covertly vandalize Wikipedia? Maybe, if they keep it very low-key. Editors are used to this kind of stuff though, it happens all the time from all governments since they can just, y'know, edit it. Anything actually impactful will be noticed by the editors which will just cause a fork.