While teaching at the U.S. Army Command and General Staff College in the 1980s, I once heard a confused student officer from a foreign country say, "I'll never understand your military. Not only does your navy have an army, but your navy's army has an air force." By navy’s army, he meant the U.S. Marine Corps, which is larger than most armies and possesses an air component far bigger than most air forces. This observation drew chuckles from the foreign officer's fellow American students, but it raised an important issue: Does the United States need two separate ground forces in a time of shrinking defense budgets?
While both the U.S. Army and Marine Corps were created during the American Revolution, until the 20th century they had very different missions. The Marines, like their European counterparts, were seaborne, protecting U.S. Navy ships against boarding and mutinies, as well as leading raiding parties. The Army's focus was frontier security and coastal defense -- in addition to cavalry patrols and garrisoning western forts, the Army manned the large fortresses guarding major East Coast ports.
Eventually, though, the distinction between the two services narrowed. During the first half of the 20th century, both undertook what today is called counterinsurgency. The Army did so in the Philippines and the Marines in Central America. In World War I, the Pacific Theater of World War II, the Korean War and the Vietnam War, Marines performed the same functions as the Army divisions that fought alongside them: large-scale conventional fighting in World War I and Korea, amphibious island-hopping operations in World War II and counterinsurgency and pacification in Vietnam.