In the years 1920, the economic advancement seems without brake; the car is accessible to the greatest number and the cinema becomes a leisure of mass. The Americans are an example for the world. But these prowesses mask many problems. The immigrants are not accommodated any more also easily; laws limit the number by nationality of it. The Protestant groups obtain the prohibition of alcohol in 1919; far from improving morality, this measurement causes a crime wave because the gangsters, such Al Capone, seek the profits drawn from clandestine distillings. The Mad years are brilliant only seemingly, but unconcern dominates.
On the international plan, the Americans did not appreciate the activism of president Wilson. They do not want to be mingled with remote quarrels any more, without concrete advantage. Also the United States does not enter the League of Nations and is satisfied to require the refunding of the war debts.
The optimism of these years is brutally confronted with the crisis of 1929. Born in the United States from an overproduction and a stock exchange speculation without brake, the Great Depression strikes the country like a catastrophe. The investments are stopped, the production slows down, unemployment takes unknown proportions: in 1932, nearly the quarter of the working population is struck by this plague. No system of compensation exists, and the famine made its appearance.
Herbert Hoover republican president is impotent vis-a-vis the extent of the phenomenon. At the time of the election of 1932, its democratic opponent, Franklin D. Roosevelt, allured by her heat and its energy; he is elected easily. Without having precise program, the new president however passes quickly to the action; its New Deal consists in starting again the production and coming to assistance of the unemployed by using all the financial resources of the state. The results are unequal, but the population is reassured. The president is re-elected triumphantly in 1936, but badly manages to suppress the crisis, which rebounds the following year. New Deal does not bring any more a solution.
In same time, the international situation is degraded in Europe and Asia, which worries Roosevelt more especially as his fellow-citizens, in their majority, are isolationist, i.e. they do not want that their country mixes with the external business. The release of the war in Europe allows in fact the United States to leave the crisis: they sell in France and Great Britain, committed only in the conflict after June 1940, supplies military and supply. But it is necessary to await the Japanese attack against Pearl Harbor, on on December 7th, 1941, so that the opinion, shocked unanimously, accepts the entry in war.
The United States quickly becomes the dominant power on the side of the Allies; they fight in the Pacific against Japan and prepare the offensive against the Nazi Germany starting from Great Britain and of North Africa. This one takes place in June 1944; the American troops release France, then penetrate in Germany to obtain its capitulation. President Roosevelt leads the negotiations with the Allies on the organization of peace to come, in particular with the conference of Yalta (February 1945).
The war, finished in May 1945 in Europe, is completed only in August in Japan, following launching by the Americans of atomic bombs on Hiroshima and Nagasaki. Meanwhile, president Roosevelt died (April 1945) and UNO was created (June 1945).