While never officially declared an Official “War” as far as the United States was concerned, the Korean Conflict,1950-53 was a Civil War in Korea whereas…”The United States responded to North Korean invasion of South Korea by going to its assistance, pursuant to United Nations Security Council resolutions. US forces deployed in Korea exceeded 300,000 during the last year of the conflict. Over 36,600 US military were killed in action…” Timeline from Wikipedia
http://en.wikipedia.org/wiki/Timeline_of_United_States_military_operations