 The invasion caught South Korea and its American ally by surprise , and sent the defending South Korean forces into retreat . The U.S. moved troops from Japan to fight alongside the South Koreans . The first troops landed on July 1 , and by July 22 , three U.S. Army divisions were in Korea , including the 1st Cavalry Division . These American troops were insufficiently trained , poorly equipped , and often led by inexperienced officers . In particular , they lacked training in how to deal with war @-@ displaced civilians . The combined U.S. and South Korean forces were initially unable to stop the North Korean advance , and continued to retreat throughout July . 
