On January 12, 1950, United States Secretary of State Dean Acheson gave a speech declaring that America’s Pacific defense perimeter was made up of the Aleutians, Ryukyu, Japan, and the Philippines, implying that America might not fight over Korea. Acheson said Korea’s defense would be the responsibility of the United Nations.
My understanding is that when the Communists went to war in Korea, they thought the US would stay out of it -- because Acheson had pretty much come out and said that.
Saying stuff because it sounds good or because you think it will make a good impression is not such a good idea when you represent the executive branch. Obama is unfit for that role because he just says anything he wants.
FWIW, my section in italics is cribbed from wikipedia.