Clearly very subjective issue, but IMO, if we had lost to an invasion (or surrendered), then Hitler would still have been defeated by the Russia. However, the US would then have had no platform to establish a western front and all of Europe could have ended up under Stalin's thumb, rather than just half of it.
The Cold War would have been very different if the USA had remained the only free democracy!
(Edited to add: I know I'm ignoring Canada, Australia, NZ and others, but you get my point!)