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the USSR (and arguably the US) proved the exact opposite by conquering entire continentsWWII demonstrated it. America learned the lesson sort of, for a while. The USSR, arguably, did not. At the end of the day, the security infrastructure the USSR had to deploy to subdue its conquered poopulations (and its nonsense with China, to say nothing of its own idiocy in Afghanistan) pretty directly lead to the economic uncompetitiveness vis-à-vis its adversaries.
And no, America wasn't running around annexing foreign territory. Where we were trying to do so indirectly, through puppets, one, that's categorically different from annexation-aimed conquest, and two, we did it clandestinely and–I'd argue–in a way that in the long term was almost never accretive to American power and interests.
Before WWII, the Mexican-American war, the German unification, the Prussian unification, Japan's conquest of Manchuria, Japan's invasion of Taiwan and the Russo-Japanese wars were each pretty definitively winning moves for the instigators.
Since WWII, the only wars of conquest that may have yielded the victor more than it cost was arguably Israel's six-day war and, for a short time, Russia's conquest of Crimea. Both subsequently undid those gains by trying to double down. (I'm not counting India's invsasion of Bangladesh as a war of conquest since nothing was annexed. Same for America and the USSR intervening everywhere without annexing.)
People keep attempting wars of conquest, or to work clandestinely and find a loophole. Nobody has. It doesn't work. It absolutely used to. Acknowledging that might help prevent empire-ending mistakes like Iraq and Iraq 2 Trump in Iran Boogaloo.