The Korean War's roots can be traced back to 1945 following the post-World War II surrender of Japan, which had occupied Korea since 1910.
After the Soviet Union declared war on Japan in August of 1945, Soviet forces invaded northern Korea, as well as other areas in Northeast Asia under Japanese control. After invading Korea, the Soviets established a major foothold in northern Korea, north of the 38th parallel — which roughly divided the Korean peninsula in half — while the United States had a strong presence in southern Korea.
In 1948, Korea officially split into what is now known as South Korea, or the Republic of Korea, and North Korea, or the Democratic People's Republic of Korea.
The Korean War began on June 25, 1950, when North Korean troops, aided by military support from China and the Soviet Union, invaded South Korea with the aim of consolidating the two Koreas into one, controlled by the communist north.
Although the Soviet Union and the United States had been allies during World War II, after the war, relations between the two nations deteriorated, ushering in what became known as the Cold War.
North Korea's invasion of South Korea was viewed unfavorably by many non-communist nations, including the U.S.
The result was that for the first time in history, the United Nations Security Council authorized the formation of a U.N. Command and dispatched troops to help repel the invaders.
The U.S. provided about 90% of the troops, but it is noteworthy that 21 other nations also participated.
At first, things were going badly for the U.S., South Korean and allied troops. North Korean forces pushed them far south into a small area around the southeastern city of Pusan (today known as Busan), which became known as the Pusan Perimeter.
On Sept. 15, 1950, U.N. forces, under the command of U.S. Army Gen. Douglas MacArthur, landed at the port of Incheon, about halfway up the peninsula on the Yellow Sea, near Seoul, South Korea's capital. The operation involved about 75,000 troops and 261 naval vessels.
The battle, which ended four days later, effectively cut the supply lines of North Korea to the south which resulted in a hasty retreat by the communists to the north and the capture of Seoul by U.N. forces.
U.N. forces then rapidly moved into North Korea, some reaching as far as the Yalu River, which lies along the border between North Korea and China.
The situation in North Korea alarmed China, and on Oct. 19, 1950, a large mass of Chinese soldiers crossed the Yalu River and, by late December, pushed U.N. forces back into South Korea.
From that time until the end of the war nearly three years later, most of the fighting centered around the 38th parallel. Seoul, which is close to the 38th parallel, was captured and then recaptured by U.N. forces a total of four times.
This post-1950 warfare, although very intense at times, is generally described as a war of attrition, with neither side gaining much ground, despite heavy casualties.
Fighting ended on July 27, 1953, when the Korean Armistice Agreement was signed. A demilitarized zone was established not far from the 38th parallel, which once again divided Korea into two nations.
Today, South Korea is a valuable ally of the U.S. Also, a large number of U.S. forces are stationed in South Korea to provide security and deter aggression.
In 1995, the Korean War Veterans Memorial was dedicated on the National Mall in Washington, D.C., to honor the millions of service members who served during the Korean War.