| | World War II ended the Great Depression of the 1930's. During the 1930's three totalitarian, militaristic powers had arisen in the world--Germany, Italy, and Japan. Germany, under Adolph Hitler and the National Socialist (Nazi) Party, invaded Poland on September 1, 1939, and Britain and France declared war upon Germany and its allies two days later. By the summer of 1940, the Nazi Blitzkrieg, or lightening war, had rolled over Denmark, Norway, the Netherlands, Belgium, and France, giving Germany control of most of western Europe. Italy declared war in June 1940, and invaded British and French Somaliland, Egypt, and Greece later that summer. Germany, Italy, and Japan signed the Tripartite Pact, making them allies, in September 1940. In the Far East, Japan had marched through China, reaching French Indochina (now Vietnam) by July 1941. The United States, however, remained neutral until December 1941. Since the close of World War I, the United States had striven to isolate itself politically from what it saw as internal European problems. President Roosevelt announced, "This nation will remain a neutral nation, but I cannot ask that every American remain neutral in thought as well." The United States remained neutral in neither thought nor action. It sold surplus weapons and traded aging destroyers to the British in exchange for military bases.
 December 1941
 On the morning of December 7, 1941, Japanese forces launched a surprise attack on the U.S. naval base at Pearl Harbor, Hawaii [see full Video]. Nineteen U.S. ships were sunk or damaged, and 3,000 Americans lost their lives. Japan also attacked Guam, Wake Island, the Philippines, and other strategic points in the Pacific at the same time Pearl Harbor was attacked. In his address to a stunned nation, President Roosevelt called December 7 "a date that will live in infamy."  full Speech & Audio versions
On December 8, the U.S. Congress declared war against Japan. Two days after Pearl Harbor, President Roosevelt told the nation, "We are going to win the war, and we are going to win the peace that follows."
September 1942
The Manhattan Project is formed to secretly build the atomic bomb before the Germans. more info
November 1942
Los Alamos is selected as the site for an atomic bomb laboratory. Robert Oppenheimer is named the director.
see our Los Alamos: Beginning of an Era section
December 1942
Fermi demonstrates the first self-sustaining nuclear chain reaction in a lab under the squash court at the University of Chicago. Soon after, a complex of top-secret nuclear production and research facilites are built by the Manhattan Project across the country.
see Fermi's account of Trinity
1942-45
The Clinton Engineer Works is built in Oak Ridge, Tennessee. It is renamed the Oak Ridge National Laboratory after World War II. The Clinton Pile, the first true plutonium production reactor, begins operation in November 1943. By March 1945, K-25 and other gaseous diffusion plants are in operation.
1943-45
The Hanford Site is built in Richland, Washington by the Manhattan Project to produce plutonium. The first reactor begins operation in September 1944.
February 1945
Yalta Summit ratifies a divided postwar Europe.
April 1945
U.S. troops liberate Nazi concentration camp at Buchenwald.
May 1945
Germany surrenders.
July 1945
The United States explodes the first atomic device at a site near Alamagordo, New Mexico.
August 1945
The United States drops atomic bombs on Hiroshima and Nagasaki. Japan surrenders.
March 1946
 Winston Churchill proclaimed an "iron curtain" had come down across Europe. By early 1946, the Soviet Union had control or influence over the Eastern European countries it had liberated from Germany. The Soviets saw this as necessary for their security, while the West saw it as aggressive. While touring the United States, former British Prime Minister Winston Churchill announced:
"From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the continent. Behind that line lie all the capitals of the ancient states of central and eastern Europe. Warsaw, Berlin, Prague, Vienna, Budapest, Belgrade, Bucharest, and Sofia, all these famous cities and the populations around them lie in the Soviet sphere and all are subject, in one form or another, not only to Soviet influence but to a very high and increasing measure of control from Moscow."
Many historians cite this speech as the formal end of the alliance between the United States, Britain, and the Soviet Union and the beginning of the cold war. Iron Curtain speech
July 1946
Atomic Energy Act (AEA) is passed, establishing the Atomic Energy Commission (AEC). The AEC replaces the Manhattan Project on December 31, 1946. The AEA places further development of nuclear technology under civilian (not military) control.
July 1946
The United States tests a nuclear bomb on Bikini Atoll, an island in the Pacific. Four days later bikini swimsuit debuts at a French fashion show.
August 1946
The Oak Ridge facility ships the first nuclear reactor-produced radioisotopes for civilian use to the Barnard Cancer Hospital in St. Louis. After World War II, Oak Ridge turns out numerous inexpensive radioactive compounds for medical diagnosis and treatment, and for research and industrial applications. 
April-May 1948
Nuclear tests in the South Pacific (Operation Sandstone) pave the way for mass production of weapons that previously had to be assembled by hand. By late 1948, the United States has 50 nuclear bombs.
June 1948
The Soviet Union begins the Berlin Blockade, cutting West Berlin off from the West. The United States begins vast airlift to keep Berlin supplied with food and fuel.
May 1949
National Chinese forces led by Chiang Kai-shek retreat from mainland China to Formosa.
August 1949
The Soviet Union detonates its first atomic device
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