Are we talking land size (USA is 4th) or population (USA is 3rd) here?
If you rate things by nominal GDP, the US is first, but the EU is a fairly close second, so the whole exceptionalism thing is isn't actually looking that exceptional.
I find it difficult to believe that there was any time after 1865 that any other single nation could have fielded a military which could have defeated a mobilized USA. What was true was that in most of those years the US was not mobilized for war, and many other nations were. But when the US was mobilized, it was stronger than any other single player.