The Army-Navy Joint Planning Committee began work in November 1938 and established five basic scenarios by June 1939. Rainbow 1 provided for the defense of the Western Hemisphere and protection of U.S. territories and trade. Rainbow 2 in addition envisaged a concerted effort with Britain and France in which the United States focused its attention on Pacific Ocean operations while its allies addressed Eastern Hemisphere conflicts. Rainbow 3 entailed the defense of the Western Hemisphere and offensive operations into the western Pacific by the United States alone. Rainbow 4 added to the defense of the Western Hemisphere the projection of troops as necessary into South America and the eastern Atlantic. Rainbow 5 considered a substantial U.S. military commitment in Europe, the eastern Atlantic, and/or Africa in concert with Britain and France.
Rainbow 2 became the focus of the planning effort when war began in Europe. Its parameters generated a far more complex military situation than the old Plan Orange for a war between the United States and Japan had considered. The involvement of European nations made it highly probable that Japanese operations to the south would spread well beyond the Philippines into Southeast Asia, the East Indies, and the southwest Pacific, and efforts to contain all of these would entail substantial American military commitments.
The success of the German offensive in western Europe by June 1940 radically changed the planning situation. War planning was predicated on, at best, German neutralization of Britain. Planning for Pacific operations (Rainbows 2 and 3) was abandoned, and the Rainbow 4 scenario was modified to accommodate the defense of the entire Western Hemisphere and U.S. possessions in the Pacific as far west as Midway and the Aleutian Islands against German, Italian, or Japanese aggression.
Continued active British resistance prompted the initiation of strategic staff conversations between the United States and Britain that stretched from 29 January to 29 March 1941. From these emerged the parameters for Rainbow 5.
Rainbow 5 assumed a conflict between the United States, the British Commonwealth, and their associated powers, on the one hand, and Germany and Italy or Germany, Italy, and Japan, on the other. The basic defensive objectives were to secure the Western Hemisphere, Britain and the British Commonwealth, and U.S. possessions against attack. The offensive objective was the defeat of Germany and its allies. To this end, the Western Allies would use all available means to apply economic pressure, conduct a sustained air offensive against Germany and its allies, seek to eliminate Italy at the earliest opportunity, and conduct limited offensives while building up substantial forces for the eventual assault on Germany. In the Pacific and Far East, Allied forces would conduct an active defense of their territories through attacks on enemy sea communications and offensives to secure the Caroline and Marshall Islands areas. Rainbow 5 was the basic American strategic plan when the United States entered the war, envisaging an active defense in the Pacific while focusing first on the defeat of Germany.
Paul E. Fontenoy
Dallek, Robert. Franklin D. Roosevelt and American Foreign Policy, 1932–1945. New York: Oxford University Press, 1979.; Matloff, Maurice, and Edwin M. Snell. Strategic Planning for Coalition Warfare, 1941–1942. Washington, DC: Center of Military History, U.S. Army, 1980.; Miller, Edward S. War Plan Orange: The U.S. Strategy to Defeat Japan, 1897–1945. Annapolis, MD: Naval Institute Press, 1991.; Ross, Steven T. American War Plans, 1941–1945: The Test of Battle. London: Cass, 1997.; Ross, Steven T. American War Plans, 1890–1939. London: Cass, 2002.