The Battle of Inchon is the correct answer.
The Battle of Inchon was an organized amphibious invasion that allowed the United Nations army, lead by McCarthur, to gain control of a key portion of North Korea near the 38th parallel.In this battle, they defeated the North Korean army.
The invasion/successful capture of this area allowed the United Nations army to recapture the capital of Seoul from the North Korean Army just a few short weeks later.
