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. Only the consecration of embattled Britain stood between him and absolute mastery of Europe- unless the United States intervened. (Manchester, 219.)
The turning point that brought about the active involvement of United States in the War was the attack of Pearl Harbor by Japan. For several years, the United States and Japan had been on a collision course over issues related to Asia. On 7 December 1941, Japanese forces struck at Pearl Harbor in Hawaii. The unexpected strike caused loss of many lives and severe damage to the air force and naval systems based there.
The attack brought about a decisive turn in the mindset of Americans about the role of US in the war. "Now the country was united as it had never been. The sneak attack, the presence of two Japanese ambassadors in Washington pretending to negotiate peace, and an old distrust of what some still called the Yellow Peril combined to transform the war into a crusade against treacherous Orientals."(Manchester, 257)
Soon after United States launched an offensive against Japan, Germany and Italy joined on the side of Japan and declared war against United States. The war in Europe was over on 7 May 1945 when Germany surrendered. Japan was crushed stage by stage under the leadership of General MacArthur and Admiral Chester Nimitz. The final blow came with the dropping of nuclear bombs in Hiroshima and Nagasaki on 6 and 8 August 1945 respectively.
The war prompted the country to consider forming a body of nations to protect peace of the world. The United Nations was formed in 1945, with United States, Soviet Union, Great Britain, China and France as the permanent members of the Security Council.
The United Nations was formed with the objective of establishing peace over the world. Paradoxically, it was here that the power struggle between the United States and Soviet Union crystallized into cold war tactics that determined much of the fate of the world for over four decades.
Many historians were perplexed by this "early and acrimonious falling-out between Moscow and Washington [who had] come together from opposite sides of the globe to beat down the most murderous and monstrous threat the modern world had