The Constitution is the answer
The Preparedness Movement was a campaign led by former Chief of Staff of the U.S. Army Leonard Wood and former President Theodore Roosevelt to strengthen the U.S. military after the outbreak of World War I.
D) To fight the spread of communism.
Because of the Domino Theory they thought that if Vietnam fell into Communism, the whole region would follow. So therefore they wanted to support South Vietnam in order to contain Communism in Asia.