United States involvement in the Cold War began with the Korean War. The war began on June 25, 1950 when North Korean troops entered the 38th parallel, which was the established post-World War II border between North Korea and South Korea. The North was supported by the Soviet Union and by the People's Republic of China, while the South was subsequently supported by the United States and its allies. Their attack was one of the first military measures of the Cold War (“Korean War”). Once North Korea invaded South Korea, US involvement took place to prevent further communist rule and their involvement aided several military developments and also left lasting political and diplomatic effects during the Cold War. At the time, U.S. participation was focused on American foreign policy. Although war did not break out until June 1950, several conflicts erupted in an attempt to subjugate the entire nation under one rule for several years after World War II. Most of these conflicts took place at the 38th parallel, where Korea was divided. Decisions influenced by President Harry S. Truman and his doctrine, which was essentially policy to contain the spread of communism, gave the United Nations the opportunity to prevent global domination through communism (“Teaching with Papers”). The powers of the Soviet Union and the People's Republic of China's fear of international communism was the main reason for the United States to intervene. In addition to preventing communism, President Truman's decision was also influenced by the apprehensive environment during The Cold War. The Soviet Union succeeded in ruining the United States as a nuclear bomb monopolist in 1949, when they successfully detonated their first… paper half… of the Korean War”). The decisive decision of the United States to enter the war was caused by the desire to govern politically and, in essence, to prevent the world domination of communist rule. The war also led to several major advances in the military, such as the use of helicopters as air ambulances and the national blood banking program that fundamentally changed blood management. Technological advances in the Korean War, “while the Russians and Americans invested heavily in technology to outdo each other in both the missile and space races,” (“The Impact of the Korean War”) had a impact on various aspects of technology, medicine and transportation. The conflict also began the first "proxy war" of the Cold War, prolonging tensions with China and instead deepening them against the Soviet Union, which began the Cold War with the division of Korea.