According to Marine Corps history, the USMC came to be on 10 November 1775. Congress did not authorize the Corps until 11 July 1798. Even so, this branch of the US Military is considered the oldest in the country, created by a group of men in Tun Tavern Philledelphia Pennsylvania.
The Marine Corps has two command paths, one going from the President to the Secretary of Defense and to the Navy, which is called Service. The other is from the President to the Defense Secretary and then to the Commandant of the Marine Corps which is called Operational. No other commander has quite the control like the Commandant does over the Corps. Marine Corps Commandants are numbered, just like kings and popes. Also, the Commandants house is the oldest in Washington DC and still serves the same purpose it did over 200 years ago.
The amount of Units in the Marine Corps is very large. The role of Marines is a generalized force in readiness. They can be deployed anywhere in the world at a moments notice and are the only military service under complete control of the President for a period of sixty days.
The Marine Corps is split into different "theaters" as you could call it. They are called a Marine Expeditionary Force and there are three of them in total. I MEF (1st Marine Expeditionary Force) is located in Camp Pendelton California. II MEF is located in Camp LeJeune North Carolina. III MEF is located in Okinawa Japan. Finally, is the Marine Corps Reserve, a unit in New Orleans Louisiana.