What started the Korean War?

The causes of the Korean War are complex, being part of both the history of Korea and the history of the Cold War. The late nineteenth and early twentieth century were periods in which Japan gradually annexed Korea, gaining control over its government. In World War II, Japan forced Koreans to enlist in its army and women (called "comfort women") to serve as prostitutes for Japanese soldiers. In face of this, a strong Korean independence movement arose. At the end of World War II, Korea obtained independence from Japan but was divided along the 38th parallel into two spheres of influence, a Soviet-aligned North Korea and United States aligned South Korea.
As the United States saw an opportunity to draw down its military presence in South Korea in the period between 1948 and 1950, North Korea, supported by China and the Soviet Union, saw an opportunity to reunify Korea as a communist country and on 25 June 1950 invaded South Korea. The United States, supported by the Security Council of the United Nations, considered this an act of war and President Truman sent US troops to support the South Korean government.