Answer: He believed in the "promise" of America being the protector of other nations. He also wanted to stop the spread of Communist domination, which he saw as an ultimate threat to U.S. security.
Explanation: President Johnson felt that there was no one else in the world who could help Vietnam at that time. He saw Communism as a movement growing in Asia that needed to be stopped so that it would not end up like the Hitler situation, which he mentioned was a lesson in history that should not be repeated. He noted that, eventually, Communism could become a danger to the U.S.A. if left alone to spread. His best statement to show these ideas he had was when he said, "...But if I left that war and let the Communists take over South Vietnam, then I would be seen as a coward and my nation would be seen as an appeaser, and we would both find it impossible to accomplish anything for anybody anywhere on the entire globe."
