There are many possible ways it could go. A nuclear conflict could devastate some parts of the world while leaving others virtually untouched.
Might that explain America's swift recovery vs. ECON's post-atomic horror in the 2070s (building the Conestoga, Valiant
and Friendship One
vs. Encounter at Farpoint
)? It always made me wonder.
Stands to reason. Even a world war can affect different parts of the world differently. People often say VGR's "Future's End" must've been in an alternate timeline without the Eugenics Wars because Los Angeles in the 1990s was intact. But none of the real wars of the 20th century had any battles fought on American soil, so why should a fictional war be any different? America is a very small percentage of Earth.
Although I think the ships you mention were the work of UESPA, not America exclusively. At least Friendship 1
was, and that tells us that United Earth had to exist as a government by then, even if it didn't include the whole world yet.