He was an experienced and well-respected general.
Due to his nearly 52 years of military training, knowledge, and experience, MacArthur was able to deduce that the Chinese were the main supporters of the North Koreans and therefore would be heavily involved in the war regardless of the weapons used against them. “Although some say ‘China was provoked into entering the war by MacArthur’s crossing the 38th parallel. MacArthur’s intelligence failed to detect Chinese troops’ movement and eventual entry on November 1950’ they are wrong because ‘Mao used the 38th crossing as pretext for entering the war. He would have entered regardless of UN orders to cross the 38th. It was only after… Stalin pressured Mao and assuring him of Soviet air support.'" MacArthur crossed the 38th parallel knowing that it would not cause the Chinese to join the war- he knew that the Chinese would join the war regardless of our invasion because Stalin and the Soviet Union was pressuring them to join in and fight. This occasion is just one of many that shows his knowledge and logic of war.
Everyone respected MacArthur and agreed with most of his war plans and so they decided to help aid him and help him capture the city of Seoul again from the communists. “It took all of MacArthur's unparalleled powers of persuasion to sell his concept to doubting Army, Navy and Marine Corps commanders. Forces gathered for the Inchon invasion included the First Marine Division, the Army's Seventh Infantry Division, some South Korean units, virtually every available amphibious ship, and dozens of other Navy warships.” MacArthur planned accordingly to fight the Chinese as he got all of the US armed forces involved- Navy, Air force, Marines, and army all played roles.