A number of historical misconceptions circulate about America’s role in the Second World War. They include the idea that America began helping Britain only when attacked herself, and that American aid to Britain before that date came in the teeth of pervasive hostility by the American public, stirred up by anti-British ‘isolationists’. The fact that America was brought fully into the war in December 1941 by the Japanese attack on Pearl Harbour, with Germany declaring war four days later, has created the false impression that America was completely separated from the war before unprovoked Axis aggression reluctantly forced the US into the conflict.
Before Pearl Harbour, America began to starve Japan of oil with an oil embargo
This imposed an enormous pressure on Japan which meant that, without huge territorial gains of oil-producing territory, Japan would run out of oil in one month. This embargo had been imposed in a large part in attempt to prevent the Japanese attacking oil-rich British territory in East Asia (what now includes Malaysia and Singapore) and also territories possessed by Britain’s ally Holland.
Hitler’s declaration of war was also the result of American help towards Britain and other allies
This assistance included $14,281 in material aid in the form of Lend Lease prior to Pearl Harbour - $1,082 million going to Britain - and direct military action. President Roosevelt had given orders that any German ship found in the eastern Atlantic should be shot at on sight. This meant the United States had more or less entered the ‘War of the Atlantic’, the crucial battle between Britain and Germany for Britain’s key sea routes. Anticipating entering the conflict, the US army grew massively from 267,767 personnel in 1940 to 1,460,998 by mid-1941 – an increase of 446%.
83% of Americans wanted Britain and France to win the war against Germany
In the same poll, taken at the beginning of the War in 1939, only 1% would admit to hoping for a German victory. It is of note that the proportion of Americans who did not express support for Britain - 17% - is very close to the percentage of the US population who were substantially of German origin, suggesting that even the dissenting minority was inspired as much by Teutonophilia as by Anglophobia.
Initial opposition to American involvement stemmed from the experience of the First World War
Initially there had been a strong public consensus that aid to the Allies was unwise (with only 20% supporting this) and this was reflected in government policy. This reflected a massive interwar consensus against American involvement in all wars – and a belief that, as in the First World War, American aid risked dragging the United States into a huge war with disastrous results. The degree of general anti-war sentiment can be seen by the fact that in January 1939 only 43% of Americans wished to defend neighbouring Mexico and 27% Brazil from attack. Support for aid to Britain rose massively as the UK suffered devastating setbacks early in the war and after Norway, the Netherlands, Luxemburg, Belgium and France fell to the Nazis. In June 1940, American opinion was split two-to-one in favour of staying out of the war even if it meant Britain losing. But by November 1940 a majority was in favour of aiding Britain even at the risk of war. By the summer of 1941, half of Americans rejected any negotiation with Germany (with 38% in support). This response suggests that much of the initial lack of support for risking war was based on the view that the Allies could win without American help. The American public consistently believed by margins of 20% or more that there would be a British victory, even during some of Britain’s darkest moments in the war - with the exception of a few months in mid 1940 when public opinion was evenly divided.
Most of America’s political leadership wanted to aid Britain and her allies
Three days before the 1940 Presidential Election, the two main candidates declared: “Our policy is to give all possible material aid to the nations which still resist aggression across the Atlantic and Pacific Oceans” (Roosevelt) and “All of us – Republicans, Democrats and Independents – believe in giving aid to the heroic British people. We must make available to them the products of our industry” (Willkie).
Franklin D. Roosevelt (U.S. President 1933-1945) obviously supported the military and other aid he pioneered. Wendell Willkie, the Republican candidate for President, owed his nomination in large part to his strong support for aiding Britain. In 1940, the Republican field for the Presidential nomination was dominated by former New York gubernatorial candidate Thomas Dewey (who was unclear in his foreign policy stances) and Ohio Senator Robert Taft (who was largely, though not entirely, anti-interventionist). Wendell Willkie was a minor candidate with less political experience than any twentieth century Republican candidate save General Eisenhower. However, his support for ‘interventionism’ was enough to sweep Willkie to the nomination over both Dewey and Taft. Nonetheless, after receiving the Republican nomination, Willkie somewhat shifted his tone on intervention, and of the two Roosevelt was clearly seen as the more likely to help Britain, with one 1940 poll finding 82% of Americans believed Roosevelt would sell naval vessels to Britain with 42.3% believing that Willkie would do so. Willkie does not appear to have benefited from this shift. Roosevelt faced low popularity levels in the late 1930s owing to domestic issues, but by 1940, election polls showed that foreign policy was Roosevelt’s greatest strength in the campaign. 
The ‘isolationists’ should not necessarily be understood as anti-British
America had only once before sent troops to Europe and isolationists sought to avoid the sort of commitments that they believed (with good cause) could lead to a massive war with Germany and (with less justification) that the United States could lose such a war. ‘Foreign orientated groups’, whether communist or Nazi, were extremely marginal among isolationists. After the declaration of War, any residual Anglophobia had no effect, with only one member of Congress (a progressive pacifist) voting against war. Charles Lindbergh, America’s most prominent isolationist, declared that had he been in Congress “I certainly would have voted for a declaration of war”. After Pearl Harbour, the leading isolationist group America First had closed down within four days and opinion polls showed a consistently large majority in favour of the war, with no organised anti war movement. During the Second World War, America was significantly influenced by and conscious of British opinion. For example British pressure played at least some role in America prioritising the German front of the war even though it was Japan that had attacked America. More than 400,000 Americans gave their lives during the war.
Strong American support for Britain has continued since the Second World War both among political elites and the general public
In February 2006, a BBC World Service poll found that Americans gave a higher rating to Britain’s effect on the world than to the United States’, with Americans believing that the United States was good for the world by 63% to 30% - and that Britain was good for the world by a 71%-14% margin. This enormous confidence was based on a long history - of which perhaps the most important was the events of the Second World War.
 Iriye, Akira (1987), The Origins of the Second World War in Asia and the Pacific, pp.147-149
 Casey, Steven (2001), Cautious Crusade: Franklin D. Roosevelt, American Public Opinion and the War against Nazi Germany, 1941-45, p.20
 Ibid, p.25
 Cole, Wayne S (1983), Roosevelt and the Isolationists, 1932-45, pp.364-365
 Casey, Steven (2001), Cautious Crusade: Franklin D. Roosevelt, American Public Opinion and the War against Nazi Germany, 1941-45, p.28
 Ibid, p.26
 Churchill, Winston S. (1949), The Second World War Volume II: Their Finest Hour, pp.488-489
 Parmet, Herbert and Marie B Hecht (1968), Never Again: A President Runs for a Third Term, New York, see particularly pp.84-88, pp.94-95, pp.100-111 and pp.161-162
 Casey, Steven (2001), Cautious Crusade: Franklin D. Roosevelt, American Public Opinion and the War against Nazi Germany, 1941-45, p.29
 Casey, Steven (2001), Cautious Crusade: Franklin D. Roosevelt, American Public Opinion and the War against Nazi Germany, 1941-45, pp.25-26
 Jonas, Manfred (1966), Isolationism in America, 1935-1941, pp.39-41
 Casey, Steven (2001), Cautious Crusade: Franklin D. Roosevelt, American Public Opinion and the War against Nazi Germany, 1941-45, p.47
 Polenberg, Richard (1972), War and Society: The United States 1941-1945, p.38 and p.134
 'CRS Report for Congress: American War and Military Operations Casualties: Lists and Statistics', Hannah Fischer, Kim Klarman, and Mari-Jana Oboroceanu, 14 May 2008, p.5 at http://www.fas.org/sgp/crs/natsec/RL32492.pdf
 'Global Poll Finds Iran Viewed Negatively', WorldPublicOpinion.org, 3 February 2006, at http://www.worldpublicopinion.org/pipa/articles/home_page/168.php?nid=&id=&pnt=168&lb=hmpg1#US