Go with: A) The Korean War.
The Potsdam Conference, held during World War II (in 1945), did not make provisions for any permanent US military presence on Asia's mainland after that war.
Following the Korean War, however, after North Korean troops invaded the South and US forces in conjunction with the United Nations worked to keep the South safe from communist encroachment, a permanent military presence by the United States was set up in South Korea. Today the US has over 20,000 military personnel in South Korea - in the United States Forces South Korea, underneath the United States Pacific Command.
