The U.S. Marine Corps is a separate service within the Department of the Navy. The Commandant of the Marine Corps is a member of the joint chiefs of staff (JCS). The Marines changed their traditional roles of providing guards of ships and naval installations and light infantry for colonial interventions by developing in the twentieth century into an amphibious force that conducts land operations essential to a naval campaign or participates in other expeditionary operations. The Corps receives much of its support from the U.S. Navy. Particularly in the twentieth century, the U.S. Marine Corps has emphasized physical fitness, intensive individual training for combat, and esprit de corps.

Although it claims lineage to the Continental Marines of the Revolutionary War, the Marine Corps began its continued existence with a congressional authorization of 11 July 1798 that established a “corps of Marines” (originally some 350 officers and enlisted personnel) headed by a Commandant, for service aboard the warships of the navy then being expanded for the Undeclared Naval War with France (1798–99). Like the British Marines, after which they were modeled, the first American Marines functioned as ships' guards and the nucleus of ships' landing parties for raids on harbors and other coastal sites.

Two centuries after its founding, the U.S. Marine Corps, with 172,200 officers and enlisted personnel in 1998, has no counterpart of comparable size and diversity among the world's armed forces. Its Fleet Marine Forces of three divisions and aircraft wings, plus other special operational units, can provide air‐ground expeditionary forces especially trained for operations from the sea, including capturing littoral objectives with amphibious assaults by surface vehicles and watercraft or helicopters. The modern Marine Corps is larger and more capable than many armies, and its aviation component, with more than 800 fighter‐attack aircraft and helicopters, is among the ten largest in the world. Although there are functional reasons for a maritime power like the United States to have such a force, the continued existence of the U.S. Marine Corps as a separate service is also a monument to the power of image, the persistence of popular and congressional support, and the unflagging belief of Marines in themselves.

The U.S. Marine Corps enjoyed no special permanence, despite its wartime origins in 1775. After the Revolution, the Continental Marines disbanded in 1783. Despite its establishment in 1798, the U.S. Marine Corps seldom exceeded 5,000 officers and men for the next 100 years. Between 1798 and 1865, its best service came as shipborne infantry and emergency cannoneers aboard American warships. When not at sea, Marines lived in barracks in navy yards to provide a guard force, sometimes joining in regional defense. The Marine Corps Act of 1834 made the Corps a distinct service within the Navy Department.

The Commandants of the Marine Corps understood that sea service had its limitations, so they stressed the readiness of barracks Marines “for such duties as the President shall direct.” These included fighting Native Americans in the Seminole Wars, quelling urban riots and small rebellions (such as John Brown's attack on the Harpers Ferry arsenal), and adding token battalions to field armies, as in the U.S. Army's capture of Veracruz and Mexico City during the Mexican War. In the Civil War, Congress considered amalgamating the Marines into the Army, but decided against it. After the war, some navy officers sought to eliminate the ships' guards and, perhaps, the entire Marine Corps. Other naval officers saw new missions for Marines in a modernized navy, including the seizure of advanced bases in the Caribbean.

U.S. expansion in the wake of the Spanish‐American War (1898) brought a new era to the Corps' development. The Marines added two new missions: the wartime task of defending advanced U.S. naval bases in the Caribbean and the Pacific; and putting small but highly trained light infantry forces behind U.S. interventions and occupations in the Caribbean, Central America, and Asia. Serving as colonial infantry in China, the Dominican Republic, Haiti, Nicaragua, Panama, and elsewhere gave the Marine Corps, which ranged between 10,000 and 18,000, a popular image of toughness, daring, and esprit de corps.

In World War I, the Marine brigade in the American Expeditionary Forces gained combat experience and new public praise. The Corps grew to 75,000. At the Battle of Belleau Wood and subsequent engagements it suffered 11,500 casualties. Marines also began to use heavy artillery and airplanes in combat.

With the decline of its role as colonial infantry, the Marine Corps turned its attention in the interwar period to creating a combined arms amphibious assault force with a central wartime mission: the seizure and defense of bases in the anticipated naval campaign against Japan in the Pacific. The Fleet Marine Force was formed in 1933 as the operational arm of the Corps, supported by Marine aviation.

During World War II, the successful war against Japan (1941–45) gave the Marine Corps a favored position in the U.S. defense establishment. The Fleet Marine Force battled its way from Guadalcanal to Okinawa, solidifying in the public mind the image of the Marine as the ultimate American warrior, thus providing the Corps with the ability to survive interservice challenges, particularly from the U.S. Army. Marines paid for the glory with some 90,000 casualties, including 19,700 killed in combat. With little administrative and logistical personnel of their own, the Marines were primarily a fighting force. Almost all Marines of World War II (a total of 669,000 men and women) served overseas. Only five percent of the U.S. armed forces, the Marine Corps suffered ten percent of all American battlefield casualties. The Marines played a vital role in the defeat of Japan, and indirectly, through creating the doctrine for amphibious landings, contributed to the defeat of Germany as well.

During the Cold War, the Marine Corps maintained its amphibious assault mission (confirmed by congressional legislation in 1947 and 1952) and added another function, the deployment to regional trouble spots of air‐ground task forces. In addition, the Corps participated in the Korean War (1950–53) and the Vietnam War (1961–75). In Korea, the Marines played pivotal roles, particularly in the 1950 Inchon landing and in the 1951 campaign that drove the Chinese from South Korea. Marines suffered 30,000 casualties in heavy fighting. As a result of the Corps' proven competence, it was authorized to double its permanent size to approximately 190,000 and to maintain three divisions and aircraft wings.

The Vietnam War showed the Marines could fight well in another extended land campaign, but at great cost. More Marines served (794,000) and more became casualties (103,000) in the Vietnam War than in World War II, in what proved to be a losing cause. By 1969, the Corps had grown to 315,000. Maintaining a Marine expeditionary force of more than two divisions and one aircraft wing in the northern five provinces of the Republic of Vietnam stretched the Corps to its limit.

In the post‐Vietnam era, internal reform helped restore public and congressional confidence in the Corps. A terrorist truck‐bombing in Lebanon killed 241 U.S. servicemen, including 220 U.S. Marines in their barracks in 1983. Participation in the intervention in Grenada (1983) and Panama (1989), the Marine Corps performed an important role in the Persian Gulf War (1991). A Marine Expeditionary Force of some 93,000 troops fought in Kuwait or held some six division of Iraqi soldiers in place along the Kuwait coast while the Allied coalition forces began their major flanking attack. Two Marine divisions breached the Kuwait border fortifications, freed the capital, and took 20,000 prisoners. Although the Marine Corps was reduced from 194,000 in 1991 to 172,200 by 1998 in the contraction of the U.S. armed forces, the Corps fended off attempts to reduce its role in the post–Cold War world.

 

It remains the nation's principal “force in readiness.”