The United States has three armed forces under the direction of the Department of Defense: the US Army, US Air Force, and US Navy. The US Marine Corps are actually a branch of the Navy, while the Coast Guard, the other armed force of the United States, is technically under the wing of the Department of Homeland Security. Each branch of the armed services in America performs a vital role for national security.
The US Marines are a highly mobile amphibious attack force. Marines are trained to attack from the water and establish a beach head, an area of control on foreign soil. After the Marines take territory, other armed forces such as the US Army move in to maintain control, while the Marines move on. Marines are mobile, lightweight, and very rapid. One might compare the Marines to the head of a spear, wedging in to get a foothold and racing ahead once the land has been secured.