The explanation is that the wars actually started under very different circumstances. In the case of Korea, a communist dictatorship invaded a capitalist dictatorship. In the case of Vietnam, the war was primarily a communist uprising within South Vietnam, with considerable support from North Vietnam. It wasn't 'democracy vs communism'; the North Vietnamese regime actually had much more popular support than the dictatorship in the South. But if you want to know how the Vietcong and North Vietnamese defeated the US and ARVN, the answer lies with the type of war that they fought. Korea was an entirely conventional war, complete with tank formations, massed infantry attacks and a LOT of artillery. Vietnam was largely a guerilla conflict. There was no single front line. By using ambush and hit-and-run tactics, the Vietcong caused the US army massive casualties despite the latter's superior equipment. You may have heard of 'search and destroy' missions and that kind of says it all; the Americans simply didn't know where the enemy was. There was no single front line as in Korea, US soldiers would be woken at night by Vietcong mortars shelling their camp. With 58,000 dead and no victory in sight, the Americans pulled out, and North Vietnam inevitably overwhelmed the South.