<P> Korea was ruled by Imperial Japan from 1910 until the closing days of World War II . In August 1945 , the Soviet Union declared war on Imperial Japan , as a result of an agreement with the United States , and liberated Korea north of the 38th parallel . U.S. forces subsequently moved into the south . By 1948 , as a product of the Cold War between the Soviet Union and the United States , Korea was split into two regions , with separate governments . Both claimed to be the legitimate government of all of Korea , and neither accepted the border as permanent . The conflict escalated into open warfare when North Korean forces -- supported by the Soviet Union and China -- moved into the south on 25 June 1950 . On 27 June , the United Nations Security Council authorized the formation and dispatch of UN forces to Korea to repel what was recognized as a North Korean invasion . Twenty - one countries of the United Nations eventually contributed to the UN force , with the United States providing 88 % of the UN 's military personnel . </P>
<P> Korea was ruled by Imperial Japan from 1910 until the closing days of World War II . In August 1945 , the Soviet Union declared war on Imperial Japan , as a result of an agreement with the United States , and liberated Korea north of the 38th parallel . U.S. forces subsequently moved into the south . By 1948 , as a product of the Cold War between the Soviet Union and the United States , Korea was split into two regions , with separate governments . Both claimed to be the legitimate government of all of Korea , and neither accepted the border as permanent . The conflict escalated into open warfare when North Korean forces -- supported by the Soviet Union and China -- moved into the south on 25 June 1950 . On 27 June , the United Nations Security Council authorized the formation and dispatch of UN forces to Korea to repel what was recognized as a North Korean invasion . Twenty - one countries of the United Nations eventually contributed to the UN force , with the United States providing 88 % of the UN 's military personnel . </P>
when did the usa join the korean war
when did the usa join the korean war
June 1950
June 1950