Distance, by itself, is not much help. The New World nations did get beaten by Spaniards who didn't have all that much of a tech advantage when all is said and done, and who were operating at the end of a very long supply line.
The key is probably that the new world nations were 1) ravaged by disease (which weakened them) and 2) politically unstable. To say the least...
Keeping that in mind, the US was weak enough in absolute terms in the 1780s that it could possibly have been conquered. (Before it became an official country, it wasn't cohesive enough to be fought, really - it was basically an insurgency.) Pushing it back from coherent nation to incoherent one in the 1780s *could* technically be done, but it's extremely tricky.
After that, the strength of the US increases quickly, and its size does so as well. The only real way to get it to be "defeatable" is to cause a major social fracture line to open up - like the slavery issue, which is probably the easiest.
If... hm. If you had a major European (Anglo-French is a favorite) intervention in the Mexican-American War, coupled with a better Mexican army in the first place, you could defeat the US quite easily. But the nations in question would have to be Draka-type to keep pressing until the point of full conquest.