What promise did MacArthur make and why was he wrong?

General Douglas MacArthur promised that the U.S. would achieve a swift victory in the Korean War and that American forces would be home quickly after defeating North Korean troops. He was wrong because he underestimated the resilience of North Korean and later Chinese forces, leading to a protracted conflict that lasted for three years and resulted in heavy casualties on both sides. Additionally, the complex geopolitical dynamics of the Cold War and the involvement of China changed the nature of the conflict, making a quick resolution impossible.