Captain Samuel Nicholas formed two battalions of Continental Marines on 10 November 1775 in Philadelphia as naval infantry. Since then, the mission of the Marine Corps has evolved with changing military doctrine and American foreign policy. The Marine Corps has served in every American armed conflict and attained prominence in the 20th century when its theories and practices of amphibious warfare proved prescient and ultimately formed the cornerstone of the Pacific campaign of World War II. By the mid-20th century, the Marine Corps had become the dominant theorist and practitioner of amphibious warfare.
In World War II, the Marines played a central role in the Pacific War. The battles of Guadalcanal, Bougainville, Tarawa, Guam, Tinian, Saipan, Peleliu, Iwo Jima, and Okinawa saw fierce fighting between Marines and the Imperial Japanese Army.
During the battle of Iwo Jima, photographer Joe Rosenthal took the famous photograph Raising the Flag on Iwo Jima of five Marines and one Navy Corpsman raising the American flag on Mt. Suribachi. Secretary of the Navy James Forrestal, having come ashore earlier that day, said of the flag-raising, "...the raising of that flag on Suribachi means a Marine Corps for the next five hundred years."