Whilst I haven't got the faintest idea of how to subjugate the United States (I know that defeating its navy, securing the West or East coast for supplies, and not treating the populace like shit might factor into it), in terms of defeat on the battlefield I believe it to be quite achievable, although most certainly not from "peer" powers, nor would the defeat be total. For example, a US Corps strength formation of about thirty thousand men is going to choke fuck a Russian or Chinese force of similar size.
I'd say, troop for troop, the most potent threat on the battlefield for the US would be a British, French or (to an extent) Japanese formation of similar size. That is an engagement the US could lose. Thusly, if that were part of a major engagement, that is a flank or main thrust that evaporates.