This entry is a subentry of World War II (1939 – 45).
The entry of the United States into World War II came formally as a consequence of the Japanese naval air attack on Pearl Harbor on 7 December 1941. But although overtly committed to neutrality, the U.S. government had been involved in the conflict, both morally and effectively, for over a year before that fateful date.
World War II was a conjunction of two geographically separated armed conflicts. The one that began with the German attack on Poland on 1 September 1939, and the British and French declarations of war on 3 September, was provoked by Adolf Hitler's commitment of the state of Germany, whose machinery he had completely captured for himself and his Nazi Party, to a drive for world hegemony. In this drive, Hitler saw, after the destruction of the Soviet Union and the annexation of its agricultural and energy resources, the United States as the ultimate enemy. Britain and France (up to the latter's defeat in 1940) recognized Hitler's drive to be incompatible with and inimical to their conception of a peaceable world based on consensus and mutual respect between the major, especially the European, powers. This conception they called “civilization,” seeing the alternative as chaos or “barbarism.” The attack on Poland was thus the occasion, not the cause, of the outbreak of armed conflict in Europe.
The second conflict arose from Japanese militarism and Japan's ambition to establish exclusive domination over greater East Asia, including Southeast Asia and the western Pacific, an ambition with strong racialist overtones. The first target of Japanese military expansionism was China, with whom conflict broke out in 1937. Britain and the United States became the second targets, since Japanese military opinion saw Western support for Nationalist China as the cause of the Chinese failure to acknowledge defeat. Britain, followed by the United States, attempted to contain Japan by diplomacy, backed by increasing economic pressure on the nation's need for imported metals and petroleum. Japan exploited German success against Britain and France to expand into Southeast Asia. U.S. economic pressure led the extreme nationalist elements in Japan to support war against the United States, accompanied by an all‐out assault on and capture of the French‐, Dutch‐, and British‐controlled areas in Southeast Asia and on the U.S.‐protected Philippines.
The two aggressor states had common enemies, including Britain and the United States—the latter, under the leadership of President Franklin D. Roosevelt, unwilling to see universalism destroyed by two cultures so antagonistic to consensual “world order” as those of Nazi Germany and militaristic‐nationalist Japan. In practice, Japan's relative weakness in the face of possible joint Anglo‐American resistance led successive Japanese governments to act only when events in Europe—the German defeat of France and occupation of the Netherlands in 1940, and the German occupation of southeastern Europe and invasion of the Soviet Union in 1941—seemed to open the way for a progressive takeover of French Indochina as an open threat to the oil, tin, rubber, and other mineral resources of Malaysia and the East Indies. President Roosevelt's decision in the winter of 1938–39 to strengthen Britain and France as the first barrier to the threat to the United States he recognized Hitler's Germany as constituting—a decision reiterated in the autumn of 1940, when British determination to continue the war against Germany was underlined by their victory in the Air Battle of Britain—made the progressive application of economic pressure on Japan inevitable. The United States was largely unprepared for war. Furthermore, the loss of British Malaysia to Japan might well prove fatal to Britain's ability to continue the war against Hitler in Europe.
Roosevelt had already shown his judgment that Germany constituted a greater threat to U.S. security than did Japan as early as 1936. To this he added a confidence in British and French military strength, suitably backed by American industry, which events were to prove quite inadequate. But apart from a moment of doubt in the summer of 1940, he never changed his policy of perceiving Germany as the main threat. The destroyers‐for‐bases agreement (1940), the Lend‐Lease Act and Agreements (1941), the denial of the western Atlantic by U.S. naval patrols to German U‐boat warfare in 1941, and the takeover of the occupation of Iceland from the British that year were all backed by a secret decision made in late 1940 that if America were to find itself at war with both Germany and Japan (wrongly believed to be coordinating their actions), priority would be given to the defeat of Germany in Europe.
American public opinion was divided, with Roosevelt somewhat restricted by isolationists, particularly in Congress. The belief that American entry into World War I had been secured by a combination of unscrupulous British propaganda, U.S. arms manufacturers, and New York bankers, bound by the scale of British purchases and borrowings to the defeat of Germany, reinforced traditional isolationism. War in Europe was a spectator sport. Most Americans supported Britain; but intervention necessitating the conscription of American youth to fight overseas was something else. Opinion on the West Coast was vigorously hostile to Japan. Yet for the bulk of Americans, war in China was likewise a spectator sport with Nationalist China the hero and militarist Japan the villain. A Japanese attack on the United States seemed inconceivable. Japan's ability to project its power across the Pacific and defeat American forces on their own territory was underestimated by both American military and civilian opinion, just as the ability of Japanese spiritual élan to overcome American technological superiority and productivity was overestimated in Japan.
By the beginning of 1941, the Japanese expansionists had already taken their moderate colleagues well on the way to war. The year 1940, believed to be the 2,600th anniversary of the founding of the Japanese empire, filled them with a millennialist, now‐or‐never spirit. In June 1940, with France defeated and invasion threatening Britain, Tokyo decided on Japanese expansion southward, rather than in China or against the Soviet Union, irrespective of any Western resistance. In August, the establishment of the “Greater East‐Asian Co‐Prosperity Sphere” was announced. In September, with American opposition hardening, Japan signed the Tripartite Pact with Fascist Italy and Nazi Germany. Direct military action forced the French to accept Japanese occupation of northern Indochina.
America stood largely aside from all this, offering little aid to the French or British, forced that summer to agree to close the route by which Nationalist China received Western arms supplies. But once Roosevelt had been reelected president in November, American economic pressure on Japan resumed. With most of its forces involved in containing Fascist Italy in the Mediterranean, Britain welcomed American leadership in the Far East, as a realization of something sought since the mid‐1930s. Staff talks between American, British, Chinese, and Dutch commanders in Southeast Asia were followed by close coordination of British and American economic pressure on Japan. The culmination was reached at the end of July 1941. The German attack on the Soviet Union on 22 June 1941 led the Japanese, freed from fears of the USSR, to occupy southern Indochina, outflanking the American‐protected Philippines and directly threatening Malaya. In response, the United States froze all Japanese assets in America and embargoed all oil and petroleum exports to Japan. Without American (or East Indian) oil, Japan's air forces and navy would be paralyzed.
There followed a dual‐track set of events for Japan. Internally, a series of imperial and ministerial/military liaison conferences drove the nation toward the decision for war. Externally, those opposed were driven to a succession of increasingly desperate attempts to negotiate an end to the embargo on terms that would not provoke a military coup in Tokyo. Had the United States leadership been willing to accept a modus vivendi by which Japan withdrew from southern Indochina in return for an end to the embargo and a free hand in China, the decision for war would have been abandoned. But neither President Roosevelt nor his secretary of state, Cordell Hull, let alone their British and Chinese allies, were ready for such a compromise, which could at best have been seen as only a postponement of conflict.
The actual Japanese decision to open hostilities by attacking Pearl Harbor only emerged as a plan in the summer of 1941 and was only adopted reluctantly in October. But it followed logically from the strategic position of the American forces in the Philippines across the flank of Japanese expansion into Malaya and the East Indies. By a narrow margin, the problem of supply was taken to rule out an actual invasion of Hawaii. U.S. strategic myopia, separate service commands, and failures in intelligence, analysis, and communications gave the Japanese total strategic and tactical surprise in their attack on Pearl Harbor; only the absent aircraft carriers, vital to America's naval victories later in 1942, escaped destruction.
President Roosevelt declared 7 December a “day of infamy,” and Congress declared war on Japan on 8 December 1941. On 11 December, under the Tripartite Pact, Hitler's Germany and Mussolini's Italy declared war on the United States. The two separate conflicts had become one global war.
