Before World War I, the Korean Empire was annexed by Japan in 1910, with the Japan-Korea Treaty of 1910. Upon the Japanese Surrender in August 1945, the U.S. Navy put the U.S. Army ashore in early September. On September 9, 1945, the Japanese forces in the southern part of Korea surrendered with ceremonies in Seoul. At the same time, Soviet Union forces occupied the northern part of Korea, with the boundary between the zones being the 38th parallel.
In the North, the Soviets supported the Korean Comunists who ruled the Democratic People's Republic of Korea (DPRK). In the South, the United States supported the Republic of Korea (ROK). Taking advantage of what seemed a decreasing task of support by the United States, DPRK's leader, Kim Il-Sung, planned to reunify Korea by force, eventually obtaining the consent from USSR and China.
On June 23, 1950, North Korea invaded South Korea. The United Nations (U.N.) brought a U.N. police action, and the United States quickly assembled to contain North Korea's army. With World War II concluding only five-years prior, the U.S. material reserves had many of the resources need with relatively modern ships, aircraft, military equipment, and production capacity. Additionally, Reserve Forces played an important role and included thousands of trained people.
The bold invasion at Inchon on September 15, 1950, broke the North Korean military progression. U.N. Armies pushed forward for the next two months before Chinese and Soviet intervention. By the middle of 1951, the front lines had stabilized near where the war started but negotiations continued for two long years, during which both sides fought on, with the U.S. Navy providing air and gunfire support, minewsweeping, and logisitic efforts.
On July 27, 1953, negotiations were concluded as the delegates attending the Armistice negotiations reached an agreement and the fighting ended. To this day, a formal peace-treaty has yet to be signed by North and South Korea.