When these two wars began, both Korea and Vietnam were divided nations with communism entrenched in the northern parts of Korea and Vietnam and anticommunism in the southern parts. President Truman’s policy resisted the spread of communism in favor of free societies that were attacked by armed minority factions or outside invaders.
The Russian and Chinese governments pushed North Korean leader Kim Il-Sung to invade South Korea and supplied troops and weapons when American forces responded to South Korea’s defense. Vietnam's war was predominately an internal civil war without external influence, except for a short period of time when China invaded in 1978. The American troops had difficulty determining friend from foe because many South Vietnamese sided with the North Vietnamese, since the South Vietnamese government was corrupt.
The U.S. did not declare war on North Korea or North Vietnam. The U.S. responded to U.N. Security Council vote and quickly built up troops to aid South Korea under Gen. Douglas MacArthur. In Vietnam, the U.S. had already supplied troops and advisers to South Vietnam when it was under French control. Johnson gradually supplied additional troops as they were needed. One goal in both conflicts was to prevent communist takeover of an anticommunist government. The "domino effect" premise proposed by Truman, and later Kennedy, was that if one government fell to communism, other countries in the area would follow.
Both wars relied heavily on bombing from aircraft, according to history professor L. Spencer Robinson, Ph.D., in an article for the American Diplomacy website. American and U.N. allied troops bombed North Korea to such an extent that most cities, supply depots and industrial sites were destroyed. The North Korean government was plunged into financial and physical ruin to such an extent that the communist forces had to negotiate a peace settlement to survive the devastation.
In Vietnam, however, the destruction from the air was not as severe. North and South Vietnamese troops were fighting the Americans, in addition to troops from China. Additionally, in Vietnam, there was no frontline warfare -- it was guerrilla warfare in a jungle that didn’t follow traditional military campaigns. The strategies used in Korea were not effective in Vietnam, according to Robinson.
The Korean War ended after three years, and the U.S. continued to aid South Korea with troops, money and supplies. Neither side was defeated, with both sides negotiating a treaty to end the hostilities. Korea remained divided after the war.
The Vietnam War lasted 10 years and wasn't one the U.S. could win, according to Robinson. President Nixon withdrew troops as the war became more unpopular and it became obvious that North Vietnamese leader Ho Chi Mihn would continue to fight until he drove the U.S. out of Vietnam, no matter how long it took. Ultimately, Vietnam reunited under communist rule.