Overview: Includes 20 illustrations. On December 7, 1941, the Japanese empire attacked the U.S. military installations in Hawaii. Four days later, Germany's dictator, Adolf Hitler, fulfilling a treaty with Japan, declared war on the United States. Having sealed with that act the developing alliance between the United States and Great Britain, Hitler's Third Reich speeded construction of a formidable "Atlantic wall," to protect the exposed beaches of the Netherlands, Belgium, and northern France. This rampart was a massive system of fortifications, obstacles, and warning centers intended to thwart an Anglo-American invasion of Nazi-occupied Western Europe. Breaching the Atlantic wall of Hitler's "Fortress Europe" was the major strategic problem confronting British and U.S. military planners in late 1941. The two Allies based their offensive strategy on the belief that Germany was the strongest of the Axis powers and therefore should be defeated first. An air offensive against Germany was an important component of this strategy. Properly conducted, it would enable the Allies to leap the Atlantic wall and damage the industrial foundations of the Third Reich well before Allied ground troops penetrated the coastal barrier. The Allies held the air forces in the Pacific and Far East to a minimum and concentrated on building a formidable force on English soil capable of striking the Nazi heartland and, eventually, of supporting a cross-channel invasion and a victorious Allied advance across Europe.