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''Military operations abroad constitute a great laboratory and proving ground for the development and testing of organization and materiel. These operations have been characterized by increasing use and importance of armored, motorized, and other specialized divisions and by concurrent effort for the development of means to counter armored (tank) divisions operating in close coordination with air and motorized units.'' General George C. Marshall

The German invasion of Poland in September 1939 marked the beginning of World War II in Europe and the beginning of the gradual end of American isolationism. As the Axis threat grew ever larger, it soon became evident that paper units would no longer suffice for the nation's defense. In this new international environment, the Army began its rebuilding program. Initially an emphasis on protection of the Western Hemisphere drove mobilization, and the victories of the German Army and Navy and their possible threat to the Americas accelerated the expansion. As events in Europe unfolded, relations between the United States and Japan deteriorated, eventually culminating in the attack on Pearl Harbor on 7 December 1941. Meanwhile, the Army steadily mustered forces, gaining fresh manpower from the first peacetime draft. Army leaders and their staffs worked overtime attempting to create an effective military force from the hollowed-out interwar Army. Along with new weapons and new concepts of warfare, new divisional tactical organizations appeared, most of which underwent several transformations before seeing combat.

In 1939 the Army created a protective mobilization force for the defense of the Western Hemisphere. The force included the 1st, 2d, 3d, 5th, and 6th Divisions, which were organized under the new triangular configuration. These forces, however, still needed to be manned and trained for war. Congressional increases in the size of the Regular Army, which began that year, provided much of the needed manpower, while the largest peacetime maneuvers ever undertaken by the Army to date provided a taste of war in 1940. Between 5 and 25 May 1940 the 1st, 2d, 5th, and 6th Infantry Divisions joined the 1st Cavalry Division, the 7th Cavalry Brigade, a provisional brigade of light and medium tanks, and other units for maneuvers near