The US military consists of five branches. They are the Army, Navy, Air Force, Marines and Coast Guard. The primary duty of all of them is to protect the security of the United States. Each of them have specific missions and functions that are independent of each other. Those different missions require different types and number of personnel. As of 2008, the US military consisted of about 3 million people. About half of them are reserve personnel.
The US Army is the oldest branch of the US military. It dates back to 1775 when the Continental Army was formed and led by General George Washington. The US Army’s primary mission is to perform land based military operations. As a result, it requires the most personnel. In 2008, the US Army had more than 1 million soldiers. 543,000 were active duty. 350,000 were in the US Army National Guard. 189,000 were in the US Army Reserve.
The US Navy also dates back to 1775 when George Washington took control of several ships to go after British supply ships and founded the Continental Navy. Its primary mission to to perform sea based military operations. Since fewer people are needed for sea based operations than land based operations, the US Navy is significantly smaller than the US Army. In 2008, the US Navy had about 460,000 sailors. About 335,000 were active duty. About 125,000 were in the US Navy Reserve.
The United States Marine Corps was formed as a sister service to the US Navy in 1775. The Continental Marines performed boarding and security operations for the US Navy. The primary mission of the USMC is to provide “force projection” from the sea. This is a very specialized mission requiring specially trained personnel. As a result, the US Marine Corps is much smaller than both the US Army and US Navy. In 2008, the USMC had about 198,000 marines. About 40,000 of them were US Marine Corps reserves.
The US Air Force became a separate branch of the US military in 1947. Prior to that it was part of the US Army. The primary mission of the USAF is to perform aerial warfare. In 2008, the US Air Force had about 400,000 personnel. About 330,000 were active duty. 74,000 were in the US Air Force Reserves. 106,000 were in the US Air National Guard.
The US Coast Guard is considered a branch of the US military even though it is in the Department of Homeland Security and not the Department of Defense. However, during wartime the USCG operates under the DOD. It was created in 1790 to protect the shoreline of the United States. It is the smallest branch of the US military. In 2008, it had about 40,000 active duty personnel.