You seem to think that war is such a simple matter. Vietnam, Korea and Iraq taught you nothing. It's never easy.

For it to be "easy", South Korea and Japan would have to let the US use bases on their territory in the war, but the risk of millions of civilian deaths will probably make them hesitant to do so early on. The moment South Korea engages in the war there will be a huge threat of Seoul being bombed, killing millions of civilians. Not involving South Korea in the long run will probably be impossible, though.

Starting out, Guam will be the American base closest to target, 3500 km away, until they can use bases in South Korea.

Yes, the US is stronger, and in the end they would win such a war, but with heavy casualties, both military and civilian.