Based on an answer submitted on Quora
There were two stages of President Franklin Roosevelt’s New Deal. The first stage, 1933–35, involving devaluing the dollar and widespread organization of cartels in order to counter extreme deflation and raise prices, was partly abandoned when major provisions of it were ruled unconstitutional. The subsequent second New Deal involved an alliance with industrial unions of the CIO that helped raise wages in major industries following a series of hard-fought strikes, including factory sit-down strikes. Collective bargaining by unions was put on firm legal foundations for the first time in US history under the National Labor Relations Act of 1935. This was popular, leading to a landslide Democratic Party victory in the 1936 elections, but unemployment remained unusually high as the second wave of the Great Depression hit in 1937; economic activity slowed down once again.
The real robust and continuous recovery of the American economy did not begin until the outbreak of World War II in Europe in September 1939. A flood of ship, raw material, and armaments orders from Britain and France soon followed. Production revved into overdrive with the fall of France to the Nazis in June 1940, when the USA’s own rearmament program took off, including the massive Two-Ocean Navy Act of July 1940, which authorized immense new warship construction that would triple the already large size of the US Navy by 1943, when all the authorized ships were finished. Aircraft production, both for Britain and for the US armed forces, was accelerated at the same time. The rapid expansion of the USArmy accelerated in September 1940 with the Selective Service Act, the first peacetime draft in American history. Not only were vast armaments now required for the rapidly expanding army, but millions of workers were drafted into the army and navy, creating job openings for the unemployed, eventually including women. Finally, the Lend Lease Act of March 1941, used US government funds to buy and “lend” massive supplies of materials and arms to the Allies, especially Britain, and, after the German invasion of the Soviet Union in June 1941, the USSR as well. Massive military construction projects for new bases and training camps also employed many, including the world’s largest office building, The Pentagon, built 1941–43. It was the enormous government-financed spending for World War II that ultimately boosted the USA out of the Great Depression to reach full employment. When Japan brought the USA direction in the war by bombing Pearl Harbor in December 1941, the economy was already accelerating toward its wartime peak in 1944. The world war truly ended the Great Depression.