The Korean War (1950-1953) was the first armed conflict of the Cold War era, and historians agree that communist North Korea would not have invaded South Korea in 1950 without the approval of Joseph Stalin, the ruthless and autocratic leader of the Soviet Union from 1922 to 1953.

But what was Stalin hoping to gain from a war in Korea? According to a letter dictated by Stalin himself months after the 1950 invasion, and discovered in Soviet archives in 2005, one of the main reasons that Stalin backed a communist invasion of South Korea was to “entangle” the United States in a costly war in East Asia and “distract” America’s attention away from Eastern Europe, Stalin’s real concern.

“Does it not give us an advantage in the global balance of power [to have America entangled in Korea]?” wrote Stalin. “It undoubtedly does.”

But while Stalin clearly wanted to portray himself as a chess grandmaster playing two steps ahead of his opponents, some historians are skeptical of the dictator’s account. It’s true that North Korea, a Soviet creation, needed Stalin’s approval to invade the South, but it’s doubtful that Stalin really intended to draw the Americans, a nuclear superpower, into the war.

More likely, say experts, is that President Harry S. Truman’s decision to send American troops to Korea took the Soviets by surprise, given that all public statements from the U.S. government (plus Soviet spy reports) indicated that America wouldn’t intervene militarily in Korea. Ahead of the invasion, Stalin secured assurances from Mao Zedong that China would send any needed reinforcements. Then, during the war, Stalin took pains to avoid Soviet forces openly engaging U.S. forces in combat.

With Japan’s defeat in World War II, it ceded control of Korea to the Allies, who agreed to divide the country roughly in half at the 38th parallel. The United States oversaw democratic elections in South Korea, while the Soviet Union installed a communist government in North Korea, known as the Democratic People’s Republic of Korea.

In 1948, Stalin and the communist leadership in the Soviet Union hand-picked 36-year-old Kim Il Sung to be the first leader of North Korea. Kim had made his name as a daring guerilla fighter and loyal communist. Interestingly, Kim got most of his military experience in the Chinese province of Manchuria, where he first led Soviet-backed Chinese forces against the Japanese, and then against Chinese Nationalists in China’s civil war.

“Kim Il Sung spoke better Russian and Chinese than he did Korean when he first took power in North Korea,” says Samuel Wells, author of Fearing the Worst: How Korea Transformed the Cold War.

While Kim was clearly an instrument of the Soviet Union, which provided key economic and military support to North Korea, he was also ambitious. Kim was eager to unify Korea under communism and he repeatedly made his case to Stalin for an invasion of South Korea throughout 1949. But Stalin, whose first priority was to avoid military conflict with the United States, initially rejected the idea.

The geopolitical balance of power shifted once again on October 1, 1949, when the communist revolutionary Mao Zedong announced the defeat of the Chinese Nationalists and the creation of the People's Republic of China (PRC). Mao was eager to strike a formal alliance with the Soviet Union, the world’s wealthiest and most powerful communist country at the time.

Mao traveled to Moscow to meet with Stalin and sign a treaty, but Wells says that the two sides clashed over the terms of the deal, until Stalin saw a way to use Korea as a bargaining chip with China.

This was all happening in early 1950, after the Truman administration had made clear that it was uninterested in sending U.S. troops to fight in Asia. Truman’s secretary of state, Dean Acheson, gave a speech on January 12, 1950, in which he specifically named the territories that were under U.S. military protection—Japan and the Philippines—and intentionally left out Korea and Taiwan, another hotly disputed territory.

This speech, plus intelligence reports from the Soviet Union’s extensive spy network, indicated that the United States wasn’t an immediate threat in Korea. But Wells thinks that Stalin wanted an additional insurance policy before he backed Kim Il Sung’s invasion of South Korea, and that insurance policy was China.

Wells says that Stalin met with Mao and agreed to China’s terms for a Sino-Soviet alliance—which included generous economic support from the Soviets—on one condition: Stalin wanted China to give Kim Il Sung approval for the invasion of South Korea. That way, if the invasion dragged on and the Americans did enter the fight, it would be China that sent in troops, not the Soviets. Mao agreed and Stalin gave Kim the green light to invade.

On June 25, 1950, the North Korean army stormed across the 38th parallel and took Seoul, the South Korean capital. The United Nations Security Council (with the Soviet Union absent) passed a resolution to send peacekeeping forces, including American troops, to defend South Korea.

In September of 1950, UN forces, led by General Douglas MacArthur, conducted a daring amphibious invasion at the coastal city of Inchon and retook Seoul. Unfortunately, that early success didn’t last. MacArthur brashly (and some say foolishly) marched his army north toward the Yalu River on the North Korean border with China, which Mao took as a direct provocation.

Mao sent hundreds of thousands of Chinese troops pouring over the border from Manchuria to overwhelm the UN and American forces, who were forced to retreat south. After the disastrous battle at the Yalu River, Americans sent in fleets of World War II-era B-29 bombers to strike targets in the North to slow the flow of Chinese reenforcements and supplies.

Stalin desperately wanted to avoid direct fighting with the United States, but he had promised Soviet air support to Mao as part of their alliance. Wells says that Stalin dragged his feet, but eventually committed a dozen regiments of Soviet MiG-15 fighter jets to defend the Chinese-North Korean border.

The MiG-15, with its swept-wing design, was incredibly fast and maneuverable, especially compared to the lumbering B-29 “Superfortress.” But even the Americans’ best fighter, the F-84 Thunderjet, was no match for the climbing speed and firepower of the MiGs. Stalin’s MiGs inflicted heavy casualties on American bombers and fighters in an area along the Chinese-North Korean border that came to be known as “MiG Alley.”

The MiG-15 was clearly a Soviet plane, but Stalin took great pains to mask direct Soviet involvement in the war. The MiGs were painted with North Korean insignia, and when Soviet pilots flew on missions, they not only dressed in Korean uniforms, but were taught basic radio commands in Korean. Any Soviet pilots shot down over UN-held areas were instructed to commit suicide rather than risk capture.

The Korean War dragged on for three years and ended with a stalemate as North and South Korea agreed to establish a demilitarized zone dividing the two countries along the 38th parallel. Stalin died a few months before the armistice was signed on July 27, 1953.

In Stalin’s 1950 letter explaining his support for the North Korean invasion, the Soviet dictator was confident that the United States would “overextend” itself in Asia, leaving a power vacuum in Europe that the Soviets could exploit. But Wells contends that the exact opposite happened.

Before the Korean War, the North Atlantic Treaty Organization (NATO) was an alliance in name only, but after the Soviet-backed invasion of South Korea, NATO created a military structure and named its first Supreme Allied Commander in 1951, General Dwight Eisenhower.

“To an extent, Stalin’s gamble backfired,” says Wells, “because it led to the creation of an actual functioning military alliance in NATO.”