American history

Second World War (1941–1945)

During the interwar period the United States again reduced its military, but mobilized to its largest levels for the ensuing Second World War. The global conflict started in the 1930's and raged until 1945, involving the majority of the world's states and every continent except Antarctica. It was the most extensive and costly armed conflict in the history of the world as well as the history of the United States.

US involvement in World War II was initially limited to providing war materiel and financial support to the United Kingdom and the Soviet Union, and isolationism remained prominent, with some American politicians calling for the US to remain out of what was perceived as a European war.

That changed instantly on 7 December 1941 with the Japanese attack on Pearl Harbor, Hawaii, followed by attacks on US and British possessions across the Pacific. An outraged President Franklin D. Roosevelt gave his famous "day of infamy" speech the next day, and Congress responded with a declaration of war on Japan.

For a few tense days, it was unclear whether the US would participate in the war in Europe, but on 11 December, the remaining Axis powers, Germany and Italy, declared war on the US, drawing the US firmly into the war and removing all doubts about the global nature of the conflict.

The loss of 8 battleships and thousands of sailors at Pearl Harbor was thought to be devastating for the US Pacific Fleet, but it forced the US to rely on its remaining aircraft carriers, which won a major victory over Japan at Midway just 6 months into the war, leading to a period of consolidation and island hopping by the Allies.

During 1942 and 1943, the US deployed millions of men and thousands of planes and tanks to the UK, beginning with strategic bombing of Nazi Germany and occupied Europe and leading up to the Allied invasions of occupied North Africa in 1942, Sicily and Italy in 1943, France in 1944, and Germany in 1945, culminating in the eventual surrender of Nazi Germany in May 1945.

In the Pacific, the US experienced much success in naval campaigns during 1944, but bloody battles at Iwo Jima and Okinawa in 1945 led the US to look for a way to end the war quickly. President Harry S Truman made the fateful decision to order the atomic bombing of Hiroshima and Nagasaki, quickly bringing about the surrender of Japan and ushering in the atomic age.

Despite the crippling effects of the Great Depression, the United States was able to mobilize quickly, eventually becoming the dominant military power in most theaters of the war (excepting only eastern Europe and mainland China), and the industrial might of the US economy is widely cited as a major factor in the Allies' eventual victory in the war.

Early in the war, the US military was perceived by some observers to be too "green" and untested to be of much use other than cannon fodder against experienced German and Japanese troops, but the US acquitted itself well and established a modern military tradition as an elite and well-trained force. Strategic and tactical lessons learned by the US, such as the importance of air superiority and the dominance of the aircraft carrier in naval actions, continue to guide US military doctrine more than 60 years later.

World War II holds a special place in the American psyche as the country's greatest triumph, and the soldiers of World War II are frequently referred to as "the greatest generation" for their sacrifices in the name of liberty. Over 16 million served (about 13% of the population), and over 400,000 were killed during the war; only the American Civil War saw more Americans killed.

The US entered the war, like many other nations, as a country struggling with economic and social problems and unsure of its identity. It emerged as one of the two undisputed superpowers along with the Soviet Union, and unlike the Soviet Union, the US homeland was virtually untouched by the ravages of war. The importance of US military and political power in world affairs since 1945 cannot be overstated; the outcome of the war and the fortunes of the victors have shaped world events to this day.

During and following the Second World War, the United States and United Kingdom developed an increasingly strong defence and intelligence relationship. Manifestations of this include extensive basing of US forces in the UK, shared intelligence, shared military technology (e.g. nuclear technology), shared procurement (mainly British purchases of American weapon systems).