On June 25, 1950, the Democratic People's Republic of Korea (North Korea) launched a surprise attack on South Korea. People in South Korea were unaware of the upcoming attack on that morning, which started a war that would kill more than 3,000,000 lives. Seoul, the capital of South Korea was quick to be captured by the North Korean troops, due to the fact that the attack had been unaccounted for.
The war had begun with North Korean leader Kim Il Song's desire to unify the peninsula under Communist rule. Before WWII, the peninsula had been one country. The UN decided to draw a line between the 38th parallel to avoid a conflict between the US and the Soviet. However, with the Soviets occupying the northern side and the US troops occupying the southern side, the two sides became increasingly different in their giverning systems.
South Korea was aided by the UN troops and the US troops, led under McArthur. North Korea held out for as long as they could, then turned to the People's Republic of China for aid. When the UN and US troops had managed to push the DPRK troops up to the border between Manchuria and North Korea, a desperate Kim Il Song turned to the Chinese. With the use of thousands of foot soldiers, the Chinese were able to push the UN and US troops back to the 38th parallel.
The result was a three-year war in the peninsula, the war ending in 1953 with an armistice.