In fact, remarkable similarities exist between the Korean War and the Vietnam War; from the US support of a dictatorial and corrupt anti-communist regime to its conception of communism as a monolithic entity, under which all communist nations were necessarily allies, rather than individuals to be dealt with separately. However, though those parallels, Vietnam era policy-makers did not apply the lessons of the Korean War to the Vietnam War. Rather, they did not seem to recognize those lessons as lessons at all, and repeated in the Vietnam War many of their previous mistakes.
The decision to engage in war in Vietnam and Korea had its ideological root in the Truman Doctrine which found clear expression in MacNamara's so called "Domino Theory". America reasoned that if first Korea and then Vietnam fell to communists, many other nations in proximity would be at risk. The US refused to have a policy of appeasement which had allowed Hitler to fortify Germany leading to WWII. In both Vietnam and Korea, America fought the forces of communism to keep nations free from Soviet control. This was the goal throughout the presidential administrations of Truman, Eisenhower, Kennedy, and Johnson. Both of these wars were fought for the same reason.
Many similarities exist between the Korean and Vietnam Wars. The main reason America fought each of these wars was to keep independent nations from succumbing to communist control. If America allowed soviets to take Korea and Vietnam, other parts of Asia would be at stake. Each case demonstrates a battle between democracy and communism. As one Vietnam veteran said, "the Korean and Vietnam Wars were fought against an ideology, not an individual you can point your finger at." Ho Chi Minh began as a nationalist fighter and only