The United States Marine Corps (USMC) serves as a versatile combat element, adapted to a wide variety of combat situations. Its original purpose was to provide naval infantry (combat forces serving aboard naval vessels), and to conduct amphibious operations from the sea onto land. The latter tactic was fully developed and utilized in World War II, most notably in the Pacific Island Campaign.
Since its inception November 10 1775, the Marine Corps has been recognized for combat prowess, and the Corps' role has been expanded significantly. Currently, the Marines serve as an all-purpose, quick-response task-force, suitable for quick insertion into areas where emergency intervention is required, and capable of utilizing ground, air, and sea elements. For example, in 1990, the 22d Marine Expeditionary Unit (22d MEU) conducted Operation Sharp Edge, a so-called NEO, or Non-combatant Evacuation Operation in the west African city of Monrovia, Liberia. Liberia was suffering from civil war at the time, and US and other civilians were not able to depart via conventional means. Sharp Edge was successful. Only one reconnaissance team came under sniper fire (no casualties either side), and several hundred civilians were evacuated with hours to US Navy vessels waiting offshore.
The Marines are unique in their mission statement, and do not necessarily fill unique combat roles. The US Army, US Navy, and US Air Force combined do overlap pretty much every area that the Marine Corps covers. However, the Marines consistantly utilize all of the essential elements of combat (air, ground, sea) together, and have perfected these tactics over the years, whereas the larger services may not work together as often, and may take some time to learn to function together in a combat theatre. The Marines do not, and should not take the place of the Army, Navy, and Air Force, any more than the ambulance takes the place of the hospital, but when an emergency situation develops and there is little time to be dealing with communications and/or political problems, the Marines essentially act as a stop-gap, to get into and hold an area until the larger machinery can be mobilized.
The Marines have one further difference from the other US military services: all marines, male or female, no matter what the occupational specialty, are trained first and foremost as riflemen. Thus the Marine Corps, at heart, is an infantry corps.
Historically, United States Marine Corps are known for several campaigns as referenced in their anthem "From the Halls of Montezuma to the Shores of Tripoli". In the 1800s, Stephen Decatur led a group of marines against the pirates of Tripoli, and separately, the Marines were known for their part in the war against Mexico.
The Marines are proud of their Gung-ho attitude and are inculcated with a strong belief in their chain of command and the importance of esprit de corps, a spirit of enthusiasm and pride in themselves and the Corps.