Korea, a small country numbering 30 million people in 1950, lies at the point where three great Asian powers (Japan, China, and the former Soviet Union) meet.
At the end of WWII, preoccupied with Soviet intentions in western Europe, the United States attached little strategic importance to Korea. America did assist South Koreans in their national elections and helped them to form the Republic of Korea (ROK).
The Soviet Union, on the other hand, took an active role in governing North Korea and in forming the Democratic People's Republic of Korea (DPRK).
The United States Army withdrew its combat forces from South Korea in 1949 but left a military advisory group to assist the South Korean Army. In early 1950, the Soviets supplied weapons to (and assigned several thousand Russian soldiers as trainers for) the North Korean People's Army.
Armed clashes between North and South Korea were common along the 38th Parallel, but in June of 1950 American observers did not anticipate an invasion of the South.
While "Western" powers were thinking it would never happen, North Korea was planning to invade South Korea in order to reunite the Korean Peninsula. The invasion began on June 25, 1950 when North Korean soldiers crossed the 38th Parallel - the dividing line between the two countries - thereby igniting a war. By September, U.S. soldiers were aiding the South as depicted in this image where they are using a bayonet tip and a screwdriver to search for hidden mines on the road from Changnyong to the Naktong River. Image online, courtesy U.S. Department of Defense.