More than three decades later, as CMC and CG FMFPac, both Marine generals were firm advocates of the force-in-readiness concept as a basic mission of the Marine Corps. It was a mission that had evolved from practice rather than theory. During the half century since the Spanish-American War, there had been only two years when U. S. Marines were not on combat duty somewhere. It had long been a tradition that the Marines, as transitory naval forces, might land on foreign soil without the implication of hostilities usually associated with invasion. This principle was invoked, along with a liberal interpretation of the Monroe Doctrine, by the State Department from 1906 to 1932 in the Caribbean and Central America. As a means of supervising unstable governments in sensitive strategic areas, Marines were sent to Cuba, Mexico, Haiti, the Dominican Republic, Nicaragua, and China for long periods of occupation.[25] [25] For a detailed discussion of this subject, see US Dept of State, Office of the Solicitor, _Right to Protect Citizens in Foreign Countries by Landing Forces_, 3d rev. ed. with sup. app. to 1933 (Washington, 1934). (State Dept Pub. No. 538.) U. S. Marines were not only web-footed infantry during these overseas operations; they also distinguished themselves as scouts, cannoneers, constabulary, engineers, and horse marines. As modern warfare grew more complex, however, the time came when the Leathernecks could no longer sail on a few hours’ notice as a “gangplank expeditionary force” made up of men detailed from the nearest posts and stations. No longer could such light weapons as machine guns, mortars, and mountain howitzers serve as the only armament necessary for seizing a beachhead. The Fleet Marine Force evolved in 1933, therefore, to fill the need for a corps of highly-trained amphibious specialists capable of carrying out a major ship-to-shore assault against modern defensive weapons. New landing craft as well as new landing tactics and techniques were developed during the next ten years, and the reputation of the Marine Corps as a force-in-readiness was upheld in the amphibious operations of World War II. During these three eventful decades of Marine development, General Cates and General Shepherd had participated in all the stages while ascending the ladder of command. Thus in the summer of 1950, they were eminently qualified for leadership in the task of building the 1st Marine Division up to war strength for the amphibious operation which General MacArthur hoped to launch on 15 September.