Why Did America Enter the Korean War?


By Patrick AllittEmory University

The Korean War intensified domestic fear of communism in the United States; there was a terrifying possibility that individuals inside America could be selling out the nation from within. With things like nuclear espionage, this was a matter of very profound concern. In fact, the Korean War put the containment policy to the test for the first time.

Photo of MacArthur, along with other army personnel, observing the attack in Korea.
General Douglas MacArthur arranged the invasion of Korea in September 1950. (Image: Nutter (Army)/Public domain)

Invasion of South Korea

North Korean dictator Kim Il-Sung invaded South Korea in June 1950. Korea was one of the many places that had been invaded by the Japanese during the Second World War, and then it had been temporarily partitioned at the 38th parallel when the Second World War ended, into a temporary communist and non-communist zone, in the expectation that it would then be reunited by elections.

A speech by Secretary of State, Dean Acheson, appeared to exclude Korea from the nations under American protection, and it was probably that which gave the North Korean dictator confidence that he could get away with an invasion.

A Supportive Foreign Policy

However, President Truman and Dean Acheson agreed that they must support the South Korean dictator, Syngman Rhee, who was an anti-communist, to prevent this invasion from succeeding.

American foreign policy after the war was strongly impregnated with the idea to fight the enemy when he’s weak and far away, even in places like Korea and Vietnam, rather than having to fight him later, when he would be stronger and closer.

This is a transcript from the video series A History of the United States, 2nd EditionWatch it now, on Wondrium.

American Invasion against North Korea

Because the Soviet Union was boycotting the United Nations Security Council due to its refusal to seat a member from communist China, the Americans were able to get a United Nations resolution condemning the North Korean invasion of the South, and supporting a United Nations military action to repel the invasion. In fact, it was overwhelmingly an American campaign, although it did have the support of the other western nations.

General Douglas MacArthur, who was presiding over the rebuilding of Japanese society on a democratic basis, took command of the American invasion.

The invasion of Korea, which MacArthur arranged in September 1950, was a technically brilliant stroke. In the space of a few months in 1950, first, the great North Korean armies flooded southward, then the American invaders enabled the Americans to push them back northward, and to follow them into North Korea.

China Joins the War

Photo of Mao Zedong
Mao Zedong sent Chinese troops to fight on behalf of the threatened North Koreans. (Image: The People’s Republic of China Printing Office/Public domain)

MacArthur’s success was so great that Mao, the leader of the Chinese revolution that had been completed the previous year, now became afraid that he might attempt to take the war to China itself. He therefore joined the war by sending Chinese troops to fight on behalf of the threatened North Koreans.

The American forces, having marched far into the north of Korea by the end of 1950, were now forced into retreat. They began to retreat back along the peninsula.

It was a very volatile period of warfare between the summer and the winter of 1950. The Americans were finally able to create an effective defensive line just north of the original dividing point on the 38th parallel, and they fought there in bitter winter conditions, as 1950 ended and 1951 began.

Arthur’s Plan to Attack China

MacArthur now advocated direct attacks on China, since it was from China that his adversaries were coming, in large part. It seemed obvious to MacArthur to attack the factories in which the enemy’s equipment was being made, and attack the enemy’s logistics, even if they were over the border in China rather than in Korea.

He also wanted to seize this opportunity to bring the nationalist Chinese under Chiang Kai Shek back into mainland China, so that they could attack China from Korea, and Chiang could attack from Taiwan—an American-backed invasion. America could roll back communism at this crucial juncture.

Evading a Nuclear Exchange

However, the Chairman of the Joint Chiefs of Staff, General Omar Bradley, and the other Joint Chiefs of Staff, in collaboration with President Truman, disagreed. They were afraid that if the war got escalated into a campaign against China, there’s every possibility that the other major communist power, the Soviet Union, would enter the war on the Chinese side, and there could possibly be nuclear exchanges over the issue in Korea.

Although Bradley and President Truman favored maintaining a non-communist zone in South Korea, they didn’t think that apocalyptic war in this place was adequate.

MacArthur Dismissed

MacArthur, dismayed by Truman’s decision in this matter, acted very rashly. He sent a letter to a sympathetic Republican congressman, who read it aloud on the floor of Congress, in which MacArthur used the phrase, “There is no substitute for victory.” This was a challenge to the president’s authority, and Truman responded by dismissing MacArthur from his command.

He was replaced by General Matthew Ridgway, who assumed command of the American troops on the ground in Korea, and was able to stabilize the situation.

Truce in Korea

Negotiations for a truce in Korea began in July 1951, but in the meantime, the fighting continued. Syngman Rhee, the South Korean leader, was trying to insist on the reunification of Korea under his leadership, and he was one of the many elements that made progress toward a truce very slow.

In fact, it wasn’t until two years later, early 1953, after Truman had retired and Eisenhower had been elected as the new president, that Eisenhower was able to go to Korea and finally bring about a truce.

Thirty-three thousand Americans had been killed in the Korean War, but it was a war of catastrophic destructiveness locally in which more than two million of the Korean people died. The demilitarized zone between North and South Korea remains, to the present, one of the tensest points of conflict between East and West, outliving even the end of the Cold War itself.

Common Questions about the United States and the Korean War

Q: Who invaded South Korea in 1950?

Kim Il-Sung, who was the North Korean dictator, invaded South Korea in June of 1950.

Q: Why did China join the Korean War?

General MacArthur’s success in pushing North Koreans back from South Korea was so great that Mao became afraid that he might attempt to take the war to China itself. He therefore joined the war.

Q: Why did President Truman oppose MacArthur’s plan to attack China?

President Truman was afraid that if the war got escalated into a campaign against China, then Soviet Union might enter the war on the Chinese side, and there could possibly be nuclear exchanges over the issue in Korea.

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