U. S. Army: Protect and defend the U.S. and its interests by way of ground troops, armor (tanks), artillery, attack helicopters and tactical nuclear weapons.
Marine Corps: Able to assault, capture, and control "beach heads" which then provide a route to attack the enemy from almost any direction. Often referred to as the "Infantry of the Navy", Marines specialize in amphibious operations.
Navy: Maintain freedom of the seas, making it possible for the U.S. to use all bodies of water when and where necessary; as well as carrying Marines to areas of conflict. Carrying an average of 80 aircraft, Navy aircraft carriers are able to deploy to areas where fixed runways are not possible. Navy submarines allow for stealth attacks on our enemies from right off their shore while Navy ships can attack targets from great distances.
U. S. Air Force: Defend the U.S. and its interests through the exploitation of air and space. To accomplish this mission the Air Force operates fighter aircraft, tanker aircraft, light and heavy bomber aircraft, transport aircraft, and helicopters; the latter of which are used mainly for rescue of downed-aircrew and special operations missions. The Air Force is also responsible for all military satellites and the control of all our strategic nuclear ballistic missiles.
Coast Guard: In peacetime, the Coast Guard is primarily concerned with law enforcement, boating safety, sea rescue, and illegal immigration control. However, the President of the United States can transfer part or all of the Coast Guard to the Department of the Navy in times of conflict.
Reserves & National Guard: Able to be tapped for trained personnel and equipment during times of need. The primary difference between the two is that the Reserves are "owned" and managed by the federal government and each state "owns" its 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.