On 25 June 1950, the might of the North Korean army rumbled across the 38th Parallel, igniting the Korean War. The United States and its South Korean allies were caught completely off guard and overwhelmed with the blitz of North Korean armor and troops flooding into South Korea. Pushed to a pocket on the southeast corner of the peninsula, near the port city of Pusan, the US and its Allies established a defensive perimeter that would hold until reinforcements from the US and the United Nations could achieve a breakout in September. The Korean War was the first major military engagement of the Cold War and the first test of the newly formed United Nations in establishing its influence over international affairs. The war would last for three years before resulting in a stalemate that has lasted until today.
The Korean peninsula has long been an integral crossroads between Japan and mainland Asia. In the First Sino-Japanese War (1894-95), the Japanese were able to destroy Chinese influence over the Koreans and help establish the short-lived Korean Empire. Following the Japanese victory over Russia in the Russo-Japanese War (1904-05), Korea was made a protectorate and then annexed in the Japanese-Korea Annexation Treaty of 1910. Korea remained under Japanese control throughout World War I and World War II, with millions of Koreans being conscripted into Japanese service as soldiers and laborers.
On 9 August 1945 the Soviet Union declared war on Japan, launching a massive invasion of Manchuria. With the Japanese war effort largely depleted from the Pacific campaign, it lacked the ability to withstand the Soviet onslaught and by 2 September surrendered to the Soviets. In the short and swift campaign, the Soviets were able to seize key territory from the Japanese, including North Korea, which would fall under the Soviet sphere of influence and establish a communist form of government in the aftermath of World War II. The Japanese defeat in China also reignited the civil war between the Communists of Mao Tse-tung and Republic of China. On 1 May 1950, the Communist Chinese were able to declare victory in that conflict, establishing a crucial supportive element for the North Koreans.
Following the successful breakout of the Pusan Perimeter, the US and its UN allies were able to push the North Koreans back across the 38th Parallel and drive to the Yalu River, which bordered China. In a surprise move, the Chinese entered the conflict supporting the North Koreans, driving the US and UN forces back south of the 38th Parallel. The two sides would continue to fight along this crucial parallel for three years, gaining little ground, reminiscent of World War I trench warfare. With the eventual stalemate, the Korea’s remained separated, in a perpetual war that has yet to see an end.