As the German Army burst across Poland in the late summer of 1939 with 14 mechanized divisions and 44 infantry and mountain divisions, the US Army could count exactly no armored divisions and six foot- and horse-mobile infantry divisions in its ranks -- and the regiments of those divisions were not even stationed together at the same posts. There were barely 190,000 men on active duty in the United States Army, and most of their training consisted of close order drill, dry firing exercises, often with outdated weapons, and mass athletics.
If the dismemberment of Czechoslovakia and the rape of Poland had not caught the American people's attention, the invasion of France and the Low Countries in the spring of 1940 did. Within a few months of the humiliation of western Europe's largest army (the French), Congress appropriated more money for the Army than it had in the previous 22 years combined. In the autumn, the National Guard was federalized and inducted into active duty around the nation, and the first peacetime draft was initiated for young American males. By the eve of the Japanese raid on Pearl Harbor, the Army had increased about 800% in size, to 1.64 million. There were 30 infantry divisions, five armored divisions, and two horse cavalry divisions on the Army rolls, of which a total of 17 divisions were rated as "combat-ready."
After the debacle at Pearl Harbor, the Army expanded to its all-time largest size, ultimately reaching 8.3 million in strength. In 1942, the United States War Department planned a ground Army of about 114 divisions of all types. Considering that the Germans possessed about 300 and the Japanese about 90 divisions, this did not seem to be an unrealistic number for a nation with as many citizens as those two principal enemy nations combined. However, several factors combined to prevent the US Army from even building 80% as many divisions as its leaders originally intended. The very different requirements for a nation isolated by two great oceans to wage war far from its shores eventually forced a much higher percentage of manpower into the Navy and Army Air Corps than in the enemy's armed forces; the losses attendant to the strategic bombing campaign were much higher than originally estimated; and the industrial skilled manpower requirements to build the aircraft and ships necessary to fight far from home also affected these plans significantly. Ultimately, the Army built only 90 divisions, of which 87 saw combat. On 15 November 1942, the 100th Infantry Division became part of the US Army's burgeoning ground combat force.