America/South Korea is mostly a historically based pairing, since they’ve only had one interaction in canon (which I’ll get to in a second).

Historically, America and South Korea have been very close allies since the 1950’s after the conclusion of WWII and the start of the Korean War. Korea had been controlled by the Japanese for pretty much the first half of the 20th century, but at the end of WWII, the Japanese in the northern half of Korea surrendered to the Soviets, and those in the southern half surrendered to the Americans, leaving the northern half of Korea under Soviet control and the southern half under American control. Each respective side, of course, tried to instill their preferred ideology of either communism or democracy. When the Korean War broke out, the US defended South Korea. Even today, there’s still something like 20 or 30-thousand US troops in South Korea, so clearly that alliance hasn’t ended. If the Korean War were to restart somehow, the US would defend South Korea again because no peace treaty was officially signed. You’ll also see stories every few months about how the US and South Korea want to hold joint military drills or war games together and North Korea starts issuing threats because they see such a thing as an act of aggression (really I think they don’t like seeing their two greatest enemies collaborating like that).