If historians were to name one decade in the 20th century as the most crucial in the emergence of the new world, few would dispute the choice of 1940s. The Second World War and the United States' involvement in it, the formation of the United Nations and the crystallization of the Cold War acted as deciding factors that changed the destinies of not only United States, but nations all over the world.
It was commonly believed that bankers eager to advance their financial interests, and munitions makers whose interests were obvious, were the real influences behind the United States' involvement in WW I. Under the circumstances, President Roosevelt reassured the nation that the United States would follow a policy of neutrality, soon after he assumed office in 1933.
However the events that followed put the country's interests at stake on several quarters. In Europe, Germany's expansion plan threatened the power balance in the region. Japan was on a similar track in Asia and this directly affected United States' interests. When Great Britain and France declared war against Germany in 1939, United States felt compelled to supply arms to these allies as a step to contain Germany's advancement in the western hemisphere:
Hitler's empire was now larger than Napoleon's, and his power was an absolute. On land he was strong enough to launch offensives in four directions simultaneously; at sea, his three hundred U-boats were strangling Britain's lifelines. ...