Welcome to the United States Army The United States Army was founded on June 14, 1775. The United States Army is the United States main fighting force originating from the Continental Army of the American Revolutionary War. The United States Army is placed under the Department of the Army, one of three military departments. The Army’s main role is ground based warfare, but it is able to respond within 18 hours of notification. The United States Army has over 400,000 active duty personnel and 540,000 Reserve and National Guard personnel. Raid Wins: Raid Loss: Operation History: