How the war started
On 25th June 1950, North Korea leader Kim Il Sung launched a surprise attack on South Korea. They were very successful. The North Korean People's Army (NKPA) easily defeated the unprepared Republic of Korea's army (the ROKs). They captured most of South Korea. As seen from the map, the brown lines represent the NKPA’s movements between June to Sept. They basically surrounded the ROKs, pushing them to Pusan.

The Americans were alarmed
On 27 June they persuaded the United Nations to pass a resolution supporting South Korea.
The Americans sent troops to Korea to reinforce the South Korean Army at Pusan and started to push the NKPA back to the 38th parallel.

-Yuxiang