On December 7, 1941, while German armies were freezing before Moscow, Japan suddenly pushed the United States into the struggle by attacking the American naval base at Pearl Harbor, Hawaii. Four days later Hitler declared war on the United States. President Roosevelt called on Congress for immediate and massive expansion of the armed forces. Twenty years of neglect and indifference, however, could not be overcome in a few days.

Helpless as American garrisons in the Pacific fell to the Japanese in the spring of 1942, military leaders in Washington worked feverishly to create a headquarters that could direct a distant war effort and to turn the fledgling ground and air units into viable, balanced fighting forces. In early 1942 the Joint Chiefs of Staff emerged as a committee of the nation's military leaders to advise the President and to coordinate strategy with the British. In March the War Department General Staff was reorganized and the Army divided into three major commands: the Air Forces, Ground Forces, and Service Forces. Thirty-seven Army divisions were in some state of training, but only one was fully trained, equipped, and deployable by January 1942. Army planners of the time estimated that victory would require an Army of nearly 9 million men, organized into 215 combat divisions, estimates that proved accurate regarding overall manpower but too ambitious for the 90 divisions that eventually were established and supported on far-flung battlefields.

Lt. Gen. Lesley J. McNair, head of Army Ground Forces and an ardent advocate of mobile war, oversaw the development of armored and airborne divisions. He directed the restructuring of existing organizations as well, turning the old World War I "square" division based on four infantry regiments into a lighter, more maneuverable triangular division with three infantry regiments. A serious and continuing shortage of Allied shipping space placed absolute limits on the size and capabilities of Army units. New tables of organization stressed leanness and mobility, sometimes at the expense of fighting power and endurance. Billeting, training areas, and equipment were all in short supply.

American industry had to support the nation's Allies as well as its own military expansion. Britain needed large amounts of munitions and equipment; and lend-lease aid, including tens of thousands of trucks and other vehicles and equipment, played an important part in mechanizing the Soviet Army. Amphibious warfare required large numbers of landing craft and support vessels, yet to be built.

The first U.S. troops arrived in the British Isles in January 1942, but nearly a year passed before they went into action against the Axis. Meanwhile, air power provided virtually the only means for the Allies to strike at Germany. The Royal Air Force began its air offensive against Germany in May 1942, and on 4 July the first American crews participated in air raids against the Continent.

In early 1942 British and American leaders reaffirmed the priority of the European theater. General Marshall argued for an immediate buildup of American forces in Great Britain, a possible diversionary attack on the Continent in the fall, and a definite full-scale invasion in 1943. The British greeted this program with caution. Remembering the enormous casualties of World War I, they preferred to strike at German power in the Mediterranean, rather than risk a direct confrontation in haste. Although acknowledging the eventual necessity for an invasion of France, they hoped to defer it until much later. Instead, Prime Minister Winston S. Churchill suggested Anglo-American landings in North Africa, bringing the French armies in France's colonies there back into the war on the side of the Allies and aiding the British in their fight against the Italians and the forces of German Field Marshal Erwin Rommel. Months of lively debate followed, but ultimately President Roosevelt directed General Marshall to plan and carry out amphibious landings on the coast of North Africa before the end of 1942.