On December 22, 1941, Britain's Prime Minister Winston Churchill and President Franklin D. Roosevelt met in Washington, D.C. for three weeks, they and their advisory shaped Anglo-American strategy for the war against the Axis powers. The two Allies agreed that Nazi Germany had to be defeated first while they fought only a holding action in the Pacific. Once the European war had been won, they would turn their combined efforts to defeating Japan.
In the two primary theaters of World War II, Europe and the Pacific, operation of entirely different character evolved, requiring munitions of different capabilities. Strategic targets in Europe included hardened submarine pens, hydroelectric dams, major transportation centers, industrial plans of all types, and POL (Petroleum, Oil, and Lubricant) facilities. Tactical targets continued to be primarily military formation and lines of communication.
As World War II spread, an increasing variety of targets came under attack by air forces, both Allied and Axis. Where as in World War I air operations were, for the most part, directed against enemy formations, in World War II, cities, industrial complexes and even populations became legitimate targets. Weapons of mass destruction were developed, including phosphorous (fire), fragmentation (antipersonnel), napalm (hardened targets), and ultimately, nuclear reaction. Delivery systems also improved.
In the Pacific Theater, operations were vastly different. Japanese industrial resources, except for POL facilities, were, for the most part, located in the home islands, and were not reachable by Allied air forces until late 1944. When they were brought under attack, it was found that fire was a more potent weapon against the relatively flimsy Japanese construction than was High Explosives. In the earlier phases of the war, offensive air operations were confined, for the most part, to enemy air and ground formation, and lines of communications; ie, airfield runways and troop and cargo shipping.