The United States Army is the main ground-force of the United States. The Army’s main function is to protect and defend the United States (and its interests) by way of ground troops, armor (tanks), artillery, attack helicopters, tactical nuclear weapons, etc.
The Army is the oldest U.S. Military service, officially established by the Continental Congress on June 14, 1775. The Army is also the largest U.S. Military Service. There are approximately 76,000 officers and 401,000 enlisted members in the active duty Army.
The Army is supported by two Reserve Forces which can be tapped for trained personnel and equipment during times of need: The Army Reserves, and the Army National Guard. The primary difference between the two is that the Reserves are “owned” and managed by the federal government, and each state “owns” it’s own National Guard. However, the President of the United States or the Secretary of Defense can “activate” state National Guard members into Federal military service during times of need.