I had a think about that one too and it's as near as makes no difference. The US basically allowed the BE in the fight, grudgingly for reasons of national prestige and kept BE forces out of critical battles. I have little doubt that had BE forces not been involved the result would have been the same
Having said that, the USSR basically cleared Japanese forces out of mainland SE Asia in the time between the first atom bomb and the Japanese surrender
I'm not competent to offer an authoritative opinion, but it seems similar to the Eastern Front: you can argue the toss about other contributions but basically one country did everything that mattered.
I agree, certainly in terms of the Pacific (as distinct from SE Asia). But given that it was overwhelmingly the atom bombs and the firebombing of Tokyo that prompted Japan's unconditional surrender, it's not really a contentious point to say the US defeated Japan effectively solo
It would also be legitimate to say that Germany defeated the French Third Republic in 1940, although Mussolini's brief but spectularly ill advised intervention might discount that on paper
I almost put 1940 down, albeit there was no lasting political settlement. Jackson's recent book on Petain's trial explores how the Nazis lucked out with Vichy but couldn't even deal the deal with them.
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(I'm not asking the question to make any kind of point; I'm genuinely interested in merely exploring the counterfactual.)
It would also be legitimate to say that Germany defeated the French Third Republic in 1940, although Mussolini's brief but spectularly ill advised intervention might discount that on paper
I suppose I'm wondering just how powerful the US really is, given the political climate.
A better general question might be: in what circumstances can a first rate power unambiguously "win"? When is that a meaningful question?