Reasons for US involvement in China?
Red China had entered the Korean War in 1950-1953. As a result of that experience, the US went out of it's way to avoid any involvement of either the Soviet Union or Red China in the Vietnam War. Red China had entered the Korean War after the US/Allied counter-attack had invaded NORTH Korea all the way to the Red Chinese border. They would most probably do that it again, if the US/Allies invaded North Vietnam all the way to the Red Chinese border.