On June 25, 1950, the Korean War (1950-1953) began when 75,000 members of the North Korean People’s Army crossed the 38th parallel and invaded South Korea. It would be the first military action of the Cold War.

In 1945, superpowers drew a line bisecting the Korean peninsula to separate the Soviet-supported Democratic People’s Republic of Korea (today’s North Korea) from the U.S.-supported Republic of Korea to the South. Essentially a civil conflict, the Korean War became a proxy war between superpowers clashing over communism and democracy. Between 2 million and 4 million people died, 70 percent of them civilians. No peace treaty was ever signed, although in December 2021, North and South Korea, the United States and China agreed to declare a formal end to the war.

What Caused the Korean War?

“The Korean War was a civil war,” says Charles Kim, Korea Foundation associate professor, Department of History, University of Wisconsin-Madison. Korea had been a unified kingdom for centuries before Japan annexed it following its victory in the Russo-Japanese War. The Japanese ruled over Korea with an iron fist from 1910 to 1945. To weaken their colony, they used assimilation tactics like forbidding the Korean language and de-emphasizing Korean history in favor of Japanese culture.

When Japan surrendered to the Allies following the dropping of atomic bombs on Hiroshima and Nagasaki during World War II, control of the Korean peninsula passed from Japan to the U.S. and the Soviet Union. The superpowers chose to divide Korea between themselves at the 38th parallel, which roughly bisected the peninsula. “It didn’t correspond to political, cultural, or terrain boundaries,” Kim says. The Soviets set up a communist government to the North, and the United States helped establish a military government in the South.

“At the time, Korean politics ran the gamut from communism on the extreme left to right-wing nationalists, all vying for power,” Kim says. “There was a lot of contention between the Soviet and U.S. occupation forces, and with the polarization of Korean leadership, it was a volatile situation,” says Kim. “Each viewed the other as illegitimate. Both wanted to invade the other to unify Korea.”

Scattered border skirmishes from 1948-50 kept tensions simmering. In 1948, the United States called on the United Nations to sponsor a vote for Koreans to determine their future government. When the North refused to participate, the South formed its own government in Seoul under the anti-communist Syngman Rhee. In retaliation, Kim Il Sung, a former communist guerrilla, was named Premier of the Democratic People’s Republic of Korea (DPRK).

Kim Il Sung went to Moscow in 1949 and again in 1950 to seek Soviet support for invading South Korea. “He was able to get Joseph Stalin to commit to providing support for the invasion of South Korea. He also got a verbal commitment from China,” Kim says.

When North Korea invaded South Korea on June 25, 1950, “North Korea was banking on the U.S. not coming back,” says Kim. North Korean forces were strong; they had the aid of experienced veterans of the Chinese Civil War, which had just ended in August of 1949. North Koreans made swift progress southward. The world watched to see what would happen next.

Why Did the U.S. Get Involved in the Korean War?

“The U.S. initially didn’t want to get involved in any kind of invasion. They didn’t want to get tangled up with North Korea, much less China or the Soviet Union,” says Kim. Key events on the world stage caused the United States to change course.

On August 29, 1949, the Soviets detonated their first atomic bomb. Klaus Fuchs, a physicist who had helped the United States build its atomic bomb program, had leaked the blueprint of the “Fat Man” atomic bomb to the Soviets. The revelation stoked Cold War paranoia. Then, on October 1, 1949, communist revolutionary Mao Zedong announced the creation of the People’s Republic of China following the defeat of the U.S.-supported Chinese nationalists led by Chiang Kai-shek. “‘The loss of China’ was a phrase used by Republican critics of the Truman administration,” says Kim.

Thousands of Chinese troops were sent to aid the North Koreans. “Mao Zedong was adamant about helping out his North Korean allies. He wanted to improve China’s prestige in the communist world by what he saw as freeing South Koreans from U.S. imperialist rule,” Kim says.

President Truman Orders US Forces to South Korea

On April 14, 1950, Truman received a document called National Security Council Paper Number 68 (NSC-68). Created by the Defense Department, the State Department, the CIA, and other agencies, it advised the president to grow the defense industry to counter what these agencies saw as the threat of global communism. The recommendations cemented Truman’s next move.

On June 27, 1950, President Truman ordered U.S. forces to South Korea to repulse the North’s invasion. “Democrats needed to look tough on communism,” Kim says. “Truman used Korea to send a message that the U.S. will contain communism and come to the aid of their allies.”

The United States never formally declared war on North Korea. Instead, Truman referred to the addition of ground troops as a “police action.” U.S. General Douglas MacArthur’s Inch’on landing on September 8, 1950, turned the tide of the war and enabled Southern forces to push Northward beyond the 38th parallel.

On December 16, 1950, U.S. President Harry Truman declared a state of emergency, proclaiming that “communist imperialism” was a threat to democracy. 

Impact of the Korean War

The Korean War armistice, signed on July 27, 1953, drew a new border between North Korea and South Korea, granting South Korea some additional territory and demilitarizing the zone between the two nations. A formal peace treaty was never signed.

Over 2.5 million people died in the Korean War. Despite two prisoner-of-war exchanges, Operation Little Switch and Operation Big Switch, 7,800 Americans are still missing in action, while South Korea is still searching for over 124,000 servicemen.

“The absence of a final conclusion to the Korean War has kept it alive as a major influence on Asian affairs,” says Sheila Miyoshi Jager, professor of East Asian Studies at Oberlin and author of Brothers at War: The Unending Conflict in Korea

She argues the Korean War directly influenced President Lyndon B. Johnson’s policy in Vietnam: “Here was a successful sovereign nation, divided by the Cold War, being threatened by its communist neighbor backed by China and the Soviet Union. Korea was now seen as a war that had successfully stopped the Chinese communist expansion in Asia.”

Sandwiched between World War II and the Vietnam War, the Korean War was nicknamed “The Forgotten War.” But to Jager, it’s not over: “The Korean War continues to influence events in East Asia,” she says. Tensions between the United States and North Korea remain.

HISTORY Vault: Korea: The Forgotten War

Five years after WWII, America is once again plunged into bloody battle. Robert Stack hosts this penetrating documentary about the war in Korea.