What can the government of South Korea do to kick the U.S. military if a war broke out between China and the U.S.?

What can the government of South Korea do to kick the U.S. military if a war broke out between China and the U.S.? Is the government of South Korea bound by a legal agreement to the whims of the U.S. military, or can the government of South Korea still kick the U.S. military out to not get caught in the crossfire?