Many supporters of U.S. involvement argued for what was known as the domino theory, a theory that believed if one country fell to communism, then the bordering countries would be sure to fall as well, much like falling dominoes. This theory was largely held due to the fall of eastern Europe to communism and the Soviet sphere of influence following World War II. However, military critics of the war pointed out that the Vietnam War was political and that the military mission lacked any clear idea of how to achieve its objectives.