The people of the United States, having rejected the Versailles Treaty and the Covenant of the League of Nations after World War I, remained largely indifferent to most international concerns. They firmly discounted the likelihood of American involvement in another major war, except perhaps with Japan. Isolationist strength in Congress led to the passage of the Neutrality Act of 1937, making it unlawful for the United States to trade with belligerents. American policy aimed at continental defense and designated the Navy as the first line of such defense. The Army's role was to serve as the nucleus of a mass mobilization that would defeat any invaders who managed to fight their way past the Navy and the nation's powerful coastal defense installations. The National Defense Act of 1920 allowed an Army of 280,000, the largest in peacetime history, but until 1939 Congress never appropriated funds to pay for much more than half of that strength. Most of the funds available for new equipment went to the fledgling air corps. Throughout most of the inter-war period, the Army was tiny and insular, filled with hard-bitten, long-serving volunteers scattered in small garrisons throughout the continental United States, Hawaii, the Philippines, and Panama.
Yet some innovative thinking and preparation for the future took place in the interwar Army. Experiments with armored vehicles and motorization, air-ground cooperation, and the aerial transport of troops came to nothing for lack of resources and of consistent high-level support. The Army did, however, develop an interest in amphibious warfare and in related techniques that were then being pioneered by the U.S. Marine Corps. By the outbreak of war the Signal Corps was a leader in improving radio communications, and American artillery practiced the most sophisticated fire-direction and -control techniques in the world. In addition, war plans for various contingencies had been drawn up, as had industrial and manpower mobilization plans. During the early 1930s Col. George C. Marshall, assistant commandant of the Infantry School at Fort Benning, Georgia, had earmarked a number of younger officers for leadership positions. Despite such preparations, the Army as a whole was unready for the war that broke out in Europe on 1 September 1939.