 The Korean War began early in the morning of 25 June 1950 , following the surprise invasion of the Republic of Korea ( ROK ) by its northern neighbour , the communist Democratic People 's Republic of Korea ( DPRK ) . Numerically superior and better @-@ equipped , the Korean People 's Army ( KPA ) crossed the 38th Parallel and rapidly advanced south , easily overcoming the South Koreans . In response , the United Nations ( UN ) decided to intervene on behalf of South Korea , inviting member states to send forces to restore the situation . As a consequence , American ground forces were hastily deployed in an attempt to prevent the South Koreans from collapsing ; however , they too were under @-@ strength and poorly equipped , and by early August had been forced back by the North Koreans to an enclave around Pusan , known as the Pusan Perimeter . Key US allies — Britain , Canada and Australia — also committed forces , although these were initially limited to naval contingents and were largely viewed as token efforts in the US . Under diplomatic pressure the British agreed to deploy an infantry brigade in July , and would later dispatch a second brigade as the crisis worsened . The Canadians also agreed to provide an infantry brigade , although the first battalion would not arrive until December 1950 . A total of 21 UN member states eventually contributed forces . 
