Question

Which of the following enabled the United States to establish a permanent military presence on the Asian mainland?

the Korean War
the Potsdam Conference
the use of atomic weapons against Japan <---- ?
the formation of the South East Asian Treaty Organization

Answer 1

Actually, the answer is A "The Korean War" :)

Answer 2

It was the "Potsdam Conference" that mostly enabled the United States to establish a permanent military presence on the Asian mainland, since this was done after it was clear that Japan was going to lose WWII to the Allies.