World War II to the German invasion of Poland in September 1939, an action that prompted Britain and France to declare war on Germany. While these events reinforced in the mind of President Roosevelt the inevitability of eventual direct US involvement in the war against Germany, the American public remained unconvinced of either the necessity or the wisdom of such involvement. The country remained, with some exceptions, isolationist, even as Roosevelt somewhat quietly increased first the sale to and later the lending of arms and other supplies to Great Britain.

The latter policy, known as “Lend-Lease” for the transition from selling supplies to essentially giving them to Great Britain for use in the war against Germany, was solidified as England suffered increasing destruction during the Battle of Britain and Paris was occupied by German troops. Throughout all of this, and the German invasion of Russia in June 1941, the American pubic remained wary of becoming involved in the growing war in Europe, but the transition in the American consciousness had begun.

Fundamental transformations in the American public began to take shape in late 1940 and during 1941. The plight of the British, details of which were transmitted daily from correspondents located in England, especially the radio broadcasts from London by American journalist Edward R. Murrow, served to galvanize public support for the British while psychologically preparing Americans for the eventuality of war. The German of Russia in June 1941 further solidified those sentiments by bringing back into the fold European and American communists and socialists who had felt betrayed by the Molotov-Ribbentrop Pact between Germany and the Soviet Union that Hitler’s and Stalin’s foreign ministers signed on August 23, 1939. Sympathies for the plight of the Russians returned among such individuals and groups and political pressure on the Roosevelt Administration, which included the American Jewish communities concerned about persecution of Europe’s Jews following the rise of Hitler, helped Roosevelt to make the case for more direct US involvement in supporting Britain.

The final development in compelling a fundamental shift in American sentiments regarding war against Germany was the Japanese attack on US military installations in Hawaii and the Japanese invasion of the Philippine Islands. The Roosevelt Administration had already imposed sanctions on Japan for its military aggression in Asia, but the December 7, 1941 attack on Pear Harbor was the event that more than any other motivated the American public to go to war. The US armed forces had been in a moribund state for much of the period between world wars, with the condition of the US Army considered almost pathetic. That changed, obviously, when the United States was directly attacked. Even the loss of ships to German submarines crossing the Atlantic with supplies for Britain failed to have the effect of the Japanese attacks that day. The result was a tremendous psychological transformation in the public that resulted in the creation of one of the most formidable military machines in history and the overwhelming sentiment of wanting to destroy Japan and Germany.

The entry of the United States into World War II marked the end of American complacency regarding European politics. The post-war world included, for the first time, a major role for the United States in international institutions and as a long-term military presence on the continent of Europe. While Americans would be divided over the following decades regarding the scale of American military presence in Europe and across Asia, the isolationism that characterized much of the public in preceding years was gone.