The United States Marine Corps was originally organized as the Continental Marines in 1775 to conduct ship-to-ship fighting, provide shipboard security and assist in landing forces. Its mission evolved with changing military doctrine and American foreign policy. Owing to the availability of Marine forces at sea, the Marine Corps has served in every conflict in U.S. history. It attained prominence when its theories and practice of amphibious warfare proved prescient, and ultimately formed a cornerstone of the Pacific campaign of World War II. By the early 20th century, the Marine Corps would become the dominant theorist and practitioner of amphibious warfare. Its ability to rapidly respond to regional crises has made and continues to make it an important tool for American foreign policy.