History.com: What Caused the Korean War and Why Did the Us Get Involved?

The Korean War (1950-1953) was the first military action of the Cold War. It was sparked by the June 25, 1950 invasion of South Korea by 75,000 members of the North Korean People's Army. The line they crossed, the 38th parallel, was created in 1945 to separate the Soviet-supported Democratic People's Republic of Korea (today's North Korea) and the U.S.-supported Republic of Korea to the South. The Korean War was a civil conflict that became a proxy war between superpowers clashing over communism and democracy. Between 2 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.