Ukraine doesn't need to retake the territory, any more than the US needed to have marines on every street corner in Iraq in order to win over there. All the Ukrainians need to do to "win" is conduct a significantly vicious and horrible bombing campaign on Russian soil. Once enough Russian civilians are killed and enough of their industry is demolished, the American paymasters will feel like they've got their money's worth and roll out the Mission Accomplished banner.

And that's it. We'll clock this as a win, on the grounds that "we spent 5% of our budget to devastate 100% of the Russian economy (don't argue or you're a Russian bot)" and inducted a bunch of new periphery countries into NATO for the purposes of staging more nuclear weapons along the borders.

Russia will be defeated because we say they've been defeated. You'll be called stupid if you suggest otherwise. And then maybe we'll be back to it in another five or ten years, when some disgruntle Ukrainian ex-pat flies a bunch of explosive-laden drones into the London Shard and President Pete Buttigieg decides that Putin was responsible.