When and why did the US get involved in WW2?

Here, Professor Evan Mawdsley shares in detail why and how the United States of America became involved in the conflict, the arguments against the USA's involvement in the 'European War', and why they remained neutral as the 1940s began…

What was the initial involvement of the USA in WW1, compared to WW2?

The beginning of WW1 was simpler than the beginning of WW2. August 1914 saw a ‘big bang’ outbreak, with the five most powerful major European powers thrown immediately into conflict. September 1939 was the beginning of a war between only three major European powers (Britain, France and Germany). They were followed in June 1940 by Italy, in June 1941 by Russia, and in December 1941 by Japan and the USA – though the conflict as a whole had actually began in July 1937 with war between China and Japan.

The American entry into World War I was also simpler than the American entry into World War II. By 1917 there was only limited opposition to taking part in the conflict. In April President Woodrow Wilson asked Congress for a declaration of war against Germany, largely in response to Berlin’s policy of unrestricted submarine warfare (sinking merchant ships without warning). The declaration was passed by 82 votes to 6 in the Senate, and by 373 to 50 in the House of Representatives. But the neutral USA entered World War II only after many months of argument in Congress and among the general public, and only when a Japanese fleet launched a big surprise air raid against its Pacific naval base at Pearl Harbor in Hawaii on 7 December 1941.

Intervention vs isolation: why didn't America want to get involved in WW2?

The outbreak of full-scale conflict between Japan and China in July 1937 had little to do with the USA and its people. Neither did Germany’s pre-war actions in the 1930s: remilitarisation, sending troops into the Rhineland in 1936, forced unification with Austria, and destruction of Czechoslovakia after the 1938 Sudenten crisis. Even Hitler’s invasion of Poland in September 1939, and the declaration of war by Britain and France seemed to have little relevance to the USA. Yet President Franklin D Roosevelt, a Democrat elected for his second term of office in November 1936, had taken a public position of opposition to aggression by Japan, Italy, and Germany. He made a speech in Chicago in October 1937, when he called for the “quarantine” of aggressive and warlike countries, and he also publicly condemned the nature of the Nazi government in Germany.

Congress, however – although both houses were controlled by the Democrats – contained strong voices opposing intervention. This opposition was based on a long-term ‘isolationist’ tradition, whose adherents did not want America – protected as it was by broad oceans – to become ‘entangled’ with foreign countries and wars overseas. Both in Congress and among the American public, this sentiment had been strengthened by the experience of World War I, which isolationists claimed America had only entered in 1917 because US manufacturers wanted to make a profit by selling munitions.

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In the 1930s many Americans did not want to become ensnared in a bloody war for the sake of distant China, or in what some Americans saw as war to perpetuate the British empire. In addition, the slow recovery from the Great Depression, with continuing high unemployment and farm problems, seemed to demand a concentration of effort on recovery at home rather than adventures abroad.

What changed America's mind about entering WW2?

The situation changed drastically in May and June 1940. Many Americans had expected the ‘European War’ to be a protracted conflict, in which the Allies – with larger populations than Germany and global resources – would eventually gain the upper hand. Instead, Adolf Hitler’s armed forces unexpectedly won quick victories, knocking France out of the war. The Third Reich took control of western and central Europe; Mussolini’s Italy opened a new front against Britain in the Mediterranean. All this threatened, perhaps in a few months, to defeat the surviving Ally. Now it seemed that unless help was provided America might have to deal on its own with a German-dominated Europe.

The implications of the May–June 1940 upheaval were global. In particular, the resource-rich southeast Asian colonies of France and the Netherlands were suddenly open to control or occupation by outsiders. Even the British colonies would be under threat if the empire’s military resources had to be concentrated in Europe and the Mediterranean. In late September 1940 this potential threat began to become a reality, when Japan sent troops into the northern French Indochina.

In the second half of 1940 the US became a vital base of support for Britain, and it grew greatly in importance as a factor in world affairs. In September 1940 Germany and neutral Japan, along with Italy, signed the Tripartite Pact. The intention was to deter the Americans from entering the war, as they had in 1917; the signatories pledged mutual support if the US went to war against any one of them. In the US public opinion was alarmed by Germany’s position in Europe; in addition, the bombing of London and other cities in the summer and autumn of 1940 and increasing submarine attacks on British ships aroused sympathy.

Roosevelt nevertheless moved cautiously; there would be a Presidential election in November 1940, and as he was running for an unprecedented third term it could be an uphill fight. He pledged to keep the US out of direct involvement in the war. In the end he took 55% of the popular vote (a decline from 61% in 1936). The Senate remained securely Democratic, but in the House of Representatives the result was only slightly better for the Democrats than in 1938, with the Roosevelt’s party winning 267 seats out of 435. The opposition Republicans won nearly 40% of the seats.

"No clear mandate for war": why did Roosevelt advocate neutrality for so long?

Victory at the polls allowed the re-elected Roosevelt more freedom of action, but not a mandate to go to war. He did not want to fight Germany and Italy without wide backing. Instead he used his presidential powers to implement a decidedly one-sided neutrality. Already in September 1940 he had provided 50 obsolete American destroyers to Britain. Now, in the winter of 1940–41 he declared that America would be an “Arsenal of Democracy”, and he succeeded by March 1941 in putting through Congress Lend-Lease legislation, providing arms to Britain without direct payment.

Secret staff talks were arranged in Washington early in 1941 between British and American military leaders. In July 1941 Marines were sent to Iceland, relieving a British garrison. In August Roosevelt and Winston Churchill staged a spectacular summit aboard warships off Newfoundland; they issued the Atlantic charter, a joint declaration opposing acts of international aggression and openly condemning Hitler and Nazism. From September the president ordered that the US Navy began escorting British convoys.

Berlin, meanwhile, strove to avoid the kind of action which had led to the US declaration of war in 1917, notably submarine attacks on US merchant ships. This restraint was motivated by plans to invade the Soviet Union, which were put into effect in June 1941; in Berlin confrontation with America was planned to come only after victory in Russia.

Even more directly important were events in the far east, where Washington became in 1941 the main counterweight to Japan. To deter Japan from occupying more Asian territory or entering the war on Germany’s side, Washington made use of powerful economic sanctions. In July 1941 the Japanese moved military forces into southern Indochina, and in response Washington froze Japanese assets and, along with the British and Dutch, cut off oil exports to Japan. American diplomats made morally understandable but diplomatically drastic demands; in the final version, the normalisation of relations and the ending of economic sanctions depended on Tokyo ending the war in China, and pulling out of the Tripartite Pact.

Some historians have seen this hard line as an attempt to provoke an attack, which would allow Roosevelt a back-door entry into a war with Germany and Italy. More reasonably, it can be suggested that Washington underestimated Japanese determination and military capabilities.

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Negotiations in Washington with Japanese diplomats continued, but in the end civilian and military leaders in Tokyo decided to seize direct control of British and Dutch resources in southeast Asia, especially the oil; only in this way could Japan’s position as a major power be maintained. In view of Roosevelt’s hard-line policy, the leaders of Japan concluded that their action would probably provoke an American entry into the war. Meanwhile the strategic position of the American-controlled Philippines had the potential to block shipping routes from southeast Asia to Japan. Tokyo decided to mount pre-emptive attacks, against both the Philippines and the American fleet at Pearl Harbor.

Pearl Harbor: did Roosevelt have knowledge ahead of the attack?

The intelligence available to Washington, mainly from communications intercepts by American codebreakers, is a complex topic. Some conspiracy theorists claimed that President Roosevelt had advance warning of the Pearl Harbor attack but let it go ahead, as the outcome would be to rally US public opinion for war; this is certainly not true. The intelligence on hand did indicate imminent Japanese action in southeast Asia (probably against Britain), but not the daring strike against Pearl Harbor.

The Pearl Harbor raid was very successful for Japan in a military sense. Politically it was idiotic; it was mounted without a declaration of war, in the middle of negotiations, on a Sunday morning, on US territory, with heavy American loss of life. As the British ambassador to Washington put it in his diary: “If war was to come with Japan I can’t imagine any way in which they could have acted more completely to rally, unite and infuriate American opinion.”

• Read more | Why didn't America see Pearl Harbor coming?

Equally remarkable was the conduct of the European Axis. Roosevelt knew from messages decrypted in the week before Pearl Harbor that Germany and Italy were now prepared to go to war with the US if Japan did so. Shrewdly, in his famous “Day of Infamy” speech before Congress on 8 December, he did not call for a declaration of war against them, but only against Japan. In this way Germany and Italy were manoeuvred into taking the first step, declaring war on 11 December.

This is sometimes described as Hitler’s greatest mistake. Germany had not been obliged to go to war with the US under the September 1940 Tripartite Pact; Tokyo had initiated the conflict, not Washington. Nevertheless, Hitler had already opted for war. He did not have advance knowledge of Pearl Harbor, but he knew that a major Japanese offensive was planned in southeast Asia; this would likely bring Britain into war against Japan, possibly with US support.

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Probably the dictator believed that Roosevelt’s hostility to the Nazi regime made war with America inevitable, and that there was little more the US could do to help Britain anyway. But the important thing was that his decision tied Germany (and Italy) to the perfidious Pearl Harbor attackers, and American public opinion rallied behind the global war. A new world order was beginning.

This article was first published by HistoryExtra in September 2019