Though it is sometimes overlooked in a cursory Western study of military history, the Korean War was a serious engagement that cost tens of thousands of US lives as well as hundreds of thousands of Korean lives. It is a big reason why North and South Korea became two countries in the first place. So how did the US end up fighting in another conflict less than five years after the devastation of WWII? What started the Korean War?
The causes of the Korean War — which began in 1950 — are complex, and the aftermath was critical. In fact, the altercations didn't end; it just went cold: The armistice that both countries signed after three years of fighting was effectively only a cease-fire, not a peace treaty. Today, the zone between North and South Korea remains the most heavily guarded border in the world.
By the end of WWII, Korea had been ruled by Japan for about 35 years. After Japan lost WWII, division of its empire fell to the Allies — namely, the US and USSR. Restoring a Korean government to the former Japanese colony was not going to be an easy process, and it was made more difficult by the fact that the US and the Soviet Union had very different ideas about what said government should be. In August of 1945, the two nations agreed to split post-WWII occupation of Korea between them, along the 38th parallel (38 degrees latitude).
The move was largely an attempt to stave off open conflict between the US and the Soviet Union in the immediate aftermath of WWII. The idea was to later unify the country under a single government whose composition would be negotiated between the US and the Soviet Union. It was very similar to the German split between east and west Germany, but ultimately led to similar problems, particularly since no Koreans had been consulted in where to draw the demarcation.
While Korea was controlled by Japan, a provisional government in exile did exist. Though Japan invaded Korea in 1910, the provisional Korean government was organized in April of 1919 in Shanghai in response to Japanese oppression during the Korean resistance known as March Movement during that same year. The provisional government was forced to conduct most of its activity in China as Japan moved to suppress all political dissidence inside Korea.
When Korea was liberated after WWII, the provisional Korean government, led by Syngman Rhee, expected to play a large role in creating a new independent and unified Korea. However, instead of the US or the Soviet Union reorganizing and supporting the skeletal organization, the Allies decided not to recognize it at all and ordered it to disband when the expatriated members returned to Korea.
In addition to the provisional government, Koreans were developing self-governing bodies by the time the US arrived in 1945. These were organized together under the Central People's Committee which proclaimed the establishment of the "Korean People's Republic" on September 6, 1945. But this organization, led by Lyuh Woon-Hyung and heavily influenced by communists, also went unrecognized by the US.
After negotiating occupational control of the southern part of Korea, the US set out to rebuild the country while the Soviet Union took over the northern half of the peninsula. The US also wanted to make their influence felt by creating an ally in mainland Asia to act as a counterweight to the Soviet Union's presence and what was rapidly becoming communist China.
The immediate problem was that, unlike the occupation of Japan, the occupation of Korea was a bit of an afterthought. The US expected the Soviet Union to insist on control of the entire Korean Peninsula, so when they agreed to split the country, the US was a little surprised. The demarcation line was drawn completely arbitrarily, with the US negotiating to keep the southern half of the peninsula so that they could retain a grip on Seoul, the country's capital. The occupation and reconstruction plan for the post-WWII Korean peninsula was essentially assembled by outsiders at the last minute.
Unlike their US counterparts in the south, the Soviet Union decided to recognize the local "people's committees" that had formed in the north. They were led by the Christian Nationalist Cho Man-sik. The right-wing leader posed a bit of a problem for the Soviet aim of creating a pro-Soviet, communist government. Cho established his own "Democratic Party" which at first, the Soviets tolerated.
Simultaneously, the Soviet Union started to encourage Korean communists living abroad to return home. One such communist was a commander of guerilla forces in Manchuria who himself escaped to the Soviet Union during the 1930s: Kim Il-sung. Having commanded forces in the Soviet Army from 1942 to 1945, Kim was a favorite of the Soviets.