America got into World War II because of Vietnam. When the Japanese conquered what was then French Indochina in September 1941, the U.S. replied with severe economic sanctions, which convinced the Japanese that America was hostile and might use its fleet to block Tokyo’s conquest of Southeast Asia. In December the Japanese attacked the U.S. fleet at Pearl Harbor.

President Dwight Eisenhower, remembering that Indochina had been a base for conquest, declared on April 7, 1954, that a communist victory there could topple the newly...