Many people today believe the Korean War was caused by the communist regime of the northern part of Korea’s desire to control the southern part of Korea, but that is only a partial secondary cause of the war. The main cause actually rested on the Soviet Union’s desire to control the entire Korean peninsula to help protect themselves from possible attack by the United States.
If you look at a map, you will see that modern day North Korea borders China and Russia. In fact, the northeastern tip of North Korea is only about 90 miles from Vladivostok, home to a large part of the Russian (then Soviet Union) Pacific Fleet. The southern tip of South Korea is only about 130 miles from Japan and 240 miles from Hiroshima, Japan. The Korean peninsula held great strategic importance to the Soviet Union and they wanted to control it through the communist regime of Kim Il-sung.
At the end of World War II, Korea was temporarily divided along the 38th parallel with the Soviet Union controlling the northern part and the US occuping the southern half, thus establishing North and South Korea.
In 1948, under United Nations supervision, South Korean held its first free elections which led to the official establishment of the Republic of Korea in South Korea. In response, the Soviet supported North Korea officially became the Democratic People’s Republic of Korea.
The Soviet Union feared that the United States would use South Korea as a base for attacking them. They wanted to drive the US out of South Korea and control the entire peninsula. Soviet leader Joseph Stalin began a military build-up in North Korea.
During this time, Stalin pressured the communist regime in China to support the military effort in North Korea. At first China resisted but eventually agreed to help support a build-up of military strength in North Korea.
On this date, June 25, 1950, the Soviet and Chinese backed government of North Korea launched a massive attack across the 38th parallel, sending 135,000 troops into South Korea, starting the Korean War. The attack caught the South Korean army and the limited US forces completely off guard.
The United States turned instantly to the United Nations for help. The UN Security Council wasted no time in voting to take military action to defend South Korea. At the time the Soviet Union had been boycotting the UN Security Council and were not present at the time of the vote. Had they been present, it’s possible the Soviet Union would have blocked UN involvement in Korea which in turn could have been a death sentence for South Korea.