As outlined in § 5063 and as originally introduced under the National Security Act of 1947, three primary areas of responsibility for the Marine Corps are:
- Seizure or defense of advanced naval bases and other land operations to support naval campaigns;
- Development of tactics, technique, and equipment used by amphibious landing forces in coordination with the Army and Air Force; and
- Such other duties as the President or Department of Defense may direct.
This last clause derives from similar language in the Congressional acts "For the Better Organization of the Marine Corps" of 1834, and "Establishing and Organizing a Marine Corps" of 1798. In 1951, the House of Representatives' Armed Services Committee called the clause "one of the most important statutory – and traditional – functions of the Marine Corps". It noted that the Corps has more often than not performed actions of a non-naval nature, including its famous actions in Tripoli, the War of 1812, Chapultepec, and numerous counter-insurgency and occupational duties (such as those in Central America), World War I, and the Korean War. While these actions are not accurately described as support of naval campaigns nor as amphibious warfare, their common thread is that they are of an expeditionary nature, using the mobility of the Navy to provide timely intervention in foreign affairs on behalf of American interests.
A U.S. Marine security guard reviews a security system at a U.S. embassy in December 2004.
The Marine Band, dubbed the "President's Own" by Thomas Jefferson, provides music for state functions at the White House. Marines from Ceremonial Companies A & B, quartered in Marine Barracks, Washington, D.C., guard presidential retreats, including Camp David, and the Marines of the Executive Flight Detachment of HMX-1 provide helicopter transport to the President and Vice President, with the radio call signs "Marine One" and "Marine Two", respectively. The Executive Flight Detachment also provides helicopter transport to Cabinet members and other VIPs. By authority of the 1946 Foreign Service Act, the Marine Security Guards of the Marine Embassy Security Command provide security for American embassies, legations, and consulates at more than 140 posts worldwide. The relationship between the Department of State and the U.S. Marine Corps is nearly as old as the corps itself. For over 200 years, Marines have served at the request of various Secretaries of State. After World War II, an alert, disciplined force was needed to protect American embassies, consulates, and legations throughout the world. In 1947, a proposal was made that the Department of Defense furnish Marine Corps personnel for Foreign Service guard duty under the provisions of the Foreign Service Act of 1946. A formal Memorandum of Agreement was signed between the Department of State and the Secretary of the Navy on 15 December 1948, and 83 Marines were deployed to overseas missions. During the first year of the MSG program, 36 detachments were deployed worldwide.
The Marine Corps was founded to serve as an infantry unit aboard naval vessels and was responsible for the security of the ship and its crew by conducting offensive and defensive combat during boarding actions and defending the ship's officers from mutiny; to the latter end, their quarters on ship were often strategically positioned between the officers' quarters and the rest of the vessel. Continental Marines manned raiding parties, both at sea and ashore. America's first amphibious assault landing occurred early in the Revolutionary War on 3 March 1776 as the Marines gained control of Fort Montague and Fort Nassau, a British ammunition depot and naval port in New Providence, the Bahamas. The role of the Marine Corps has expanded significantly since then; as the importance of its original naval mission declined with changing naval warfare doctrine and the professionalization of the naval service, the Corps adapted by focusing on formerly secondary missions ashore. The Advanced Base Doctrine of the early 20th century codified their combat duties ashore, outlining the use of Marines in the seizure of bases and other duties on land to support naval campaigns.
Throughout the late 19th and 20th centuries, Marine detachments served aboard Navy cruisers, battleships and aircraft carriers. Marine detachments served in their traditional duties as a ship's landing force, manning the ship's weapons and providing shipboard security. Marine detachments were augmented by members of the ship's company for landing parties, such as in the First Sumatran Expedition of 1832, and continuing in the Caribbean and Mexican campaigns of the early 20th centuries. Marines would develop tactics and techniques of amphibious assault on defended coastlines in time for use in World War II. During World War II, Marines continued to serve on capital ships. They often were assigned to man anti-aircraft batteries. When gun cruisers were retired by the 1960s, the remaining Marine detachments were only seen on battleships and carriers. Its original mission of providing shipboard security finally ended in the 1990s.
The Marine Corps fulfills a critical military role as an amphibious warfare force. It is capable of asymmetric warfare with conventional, irregular, and hybrid forces.
While the Marine Corps does not employ any unique capabilities, as a force it can rapidly deploy a combined-arms task force to almost anywhere in the world within days. The basic structure for all deployed units is a Marine Air-Ground Task Force (MAGTF) that integrates a ground combat element, an aviation combat element and a Logistics Combat Element under a common command element. While the creation of joint commands under the Goldwater–Nichols Act has improved inter-service coordination between each branch, the Corps' ability to permanently maintain integrated multi-element task forces under a single command provides a smoother implementation of combined-arms warfare principles.
The close integration of disparate Marine units stems from an organizational culture centered on the infantry. Every other Marine capability exists to support the infantry. Unlike some Western militaries, the Corps remained conservative against theories proclaiming the ability of new weapons to win wars independently. For example, Marine aviation has always been focused on close air support and has remained largely uninfluenced by air power theories proclaiming that strategic bombing can single-handedly win wars.
This focus on the infantry is matched with the doctrine of "Every Marine [is] a rifleman", a focus of Commandant Alfred M. Gray, Jr., emphasizing the infantry combat abilities of every Marine. All Marines, regardless of military specialization, receive training as a rifleman; and all officers receive additional training as infantry platoon commanders. For example, at Wake Island, when all of the Marine aircraft were destroyed, pilots continued the fight as ground officers, leading supply clerks and cooks in a final defensive effort. Flexibility of execution is implemented via an emphasis on "commander's intent" as a guiding principle for carrying out orders; specifying the end state but leaving open the method of execution.
The amphibious assault techniques developed for World War II evolved, with the addition of air assault and maneuver warfare doctrine, into the current "Operational Maneuver from the Sea" doctrine of power projection from the seas. The Marines are credited with the development of helicopter insertion doctrine and were the earliest in the American military to widely adopt maneuver-warfare principles, which emphasize low-level initiative and flexible execution. In light of recent warfare that has strayed from the Corps' traditional missions, it has renewed an emphasis on amphibious capabilities.
Machine gunner from 1st Platoon, Company B, 1st Battalion, 23rd Marine Regiment in cold weather gear armed with a Squad Automatic Weapon taking part in a security patrol around Ramadi, Iraq, 27 December 2006.
The Marine Corps relies on the Navy for sealift to provide its rapid deployment capabilities. In addition to basing a third of the Fleet Marine Force in Japan, Marine Expeditionary Units (MEU) are typically stationed at sea so they can function as first responders to international incidents. To aid rapid deployment, the Maritime Pre-Positioning System was developed: fleets of container ships are positioned throughout the world with enough equipment and supplies for a Marine Expeditionary Force to deploy for 30 days.
The USMC is planning to reduce its logistical requirements and by 2025 eliminate all liquid fuel use for Marine Expeditionary Forces, except for highly efficient vehicles.
Two small manuals published during the 1930s would establish USMC doctrine in two areas. The Small Wars Manual laid the framework for Marine counter-insurgency operations from Vietnam to Iraq and Afghanistan while the Tentative Landing Operations Manual established the doctrine for the amphibious operations of World War II. "Operational Maneuver from the Sea" is the current doctrine of power projection.