After a slow mobilization, the United States of America helped bring about a decisive victory by supplying badly needed financing, food, and millions of fresh and eager soldiers.

In foreign policy President Wilson helped found the League of Nations but the U.S. never joined it, as the Congress refused to give up its constitutional role in declaring war.

By 1939, isolationist sentiment in America had ebbed, and after the stunning fall of France in 1940 to Nazi Germany, the United States began rearming itself and sent a large stream of money and military supplies to Britain and the Soviet Union.

Following the attack on Pearl Harbor, the United States entered World War II to fight against Nazi Germany, Fascist Italy, and Imperial Japan, known as the "Axis Powers".

Firmly maintaining neutrality when World War I began in Europe in 1914, the United States helped supply the Allies, but could not ship anything to the Central Powers because of the British blockade of Germany.

German efforts to use their submarines ("U-boats") to blockade Britain resulted in the deaths of American travelers and sailors, and attacks on passenger liners caused public outrage.

Germany promised not to repeat; however it reversed position in early 1917, believing that unrestricted U-boat warfare against all ships headed to Britain would win the war, albeit at the cost of American entry.

The solution included nationalizing the coal mines and the railroads for the duration, shutting down factories one day a week to save fuel, and enforcing a strict system of priorities.

"[15] A popular Tin Pan Alley song of 1919 asked, concerning the United States troops returning from World War I, "How Ya Gonna Keep 'em Down on the Farm (After They've Seen Paree)?".

Under the multi-national Dawes Plan, American and British banks loaned money to Germany to stabilize its currency and expand production, which then allowed it to pay reparations to countries like Britain and France.

By 1920, over 10,000 arrests were made, and the immigrants caught up in these raids were deported back to Europe, most notably the anarchist Emma Goldman, who years before had attempted to assassinate industrialist Henry Clay Frick.

Harding used new advertising techniques to lead the GOP to a massive landslide, carrying the major cities as many Irish Catholics and Germans, feeling betrayed, temporarily deserted the Democrats.

[27] With the oil booms in Texas, Oklahoma, and California, the United States dominated world petroleum production, now even more important in an age of automobiles and trucks.

[29] The promise of prosperity made Hoover president, but the reality of economic decline ruined him, as Democrats tarred him with blame for the Great Depression in the United States.

The unions held on to their gains among machinists, textile workers, and seamen, and in such industries as food and clothing, but overall membership fell back to 3.5 million, where it stagnated until the New Deal passed the Wagner Act in 1935.

[35] The "Jazz Age" symbolized the popularity of new musics and dance forms, which attracted younger people in all the large cities as the older generation worried about the threat of looser sexual standards as suggested by the uninhibited "flapper."

The KKK of the 1920s was a purification movement that rallied against crime, especially violation of prohibition, and decried the growing "influence" of "big-city" Catholics and Jews, many of them immigrants and their descendants from Ireland as well as Southern and Eastern Europe.

In a spectacular trial that drew national attention thanks to the roles of three-time Democratic presidential candidate William Jennings Bryan for the prosecution and famed lawyer Clarence Darrow for the defense, John T. Scopes was convicted of teaching evolution, but the verdict was overturned on a technicality.

Conducted outside the auspice of the League of Nations, it was attended by nine nations—the United States, Japan, China, France, Great Britain, Italy, Belgium, Netherlands, and Portugal[51] Russia and Germany were pariahs and were not invited.

[53] Under the plan, developed by a committee headed by American Charles G. Dawes, New York and London banks loaned Germany hundreds of millions of dollars to stabilize its currency and expand production, which then allowed it to pay reparations.

[60] In the 1930s, the United States entered the period of deep isolationism, rejecting international conferences, and focusing moment mostly on reciprocal tariff agreements with smaller countries of Latin America.

Unlike the loans in World War I, the United States made large-scale grants of military and economic aid to the Allies through Lend-Lease, with little expectation of repayment.

[66] By 1940 they were attracted by booming munitions factories in plants in the Great Lakes region Nationwide, farmers had been experiencing depressed market conditions for their crops and goods since the end of World War I.

[67] In the United States, upon accepting Democratic nomination for president in 1932, Franklin D. Roosevelt promised "a new deal for the American people," a phrase that has endured as a label for his administration and its many domestic achievements.

[71] The desperate economic situation, combined with the substantial Democratic victories in the 1932 Congressional elections, gave Roosevelt unusual influence over Congress in the "First Hundred Days" of his administration.

The bill provided for United States Treasury Department inspection of all banks before they would be allowed to reopen, for federal assistance to tottering large institutions, and for a thorough reorganization of those in greatest difficulty.

The celebrated First Hundred Days of the new administration also produced a federal program to protect American farmers from the uncertainties of the market through subsidies and production controls, the Agricultural Adjustment Act (AAA), which Congress passed in May 1933.

By 1943 one needed government issued ration coupons to purchase typewriters, sugar, gasoline, bicycles, footwear, fuel oil, silk, nylon, coffee, stoves, shoes, meat, cheese, butter, margarine, canned foods, dried fruits, jam, and many other items.

[90] The unemployment problem ended in the United States with the beginning of World War II, when stepped up wartime production created millions of new jobs and the draft pulled young men out of the labor pool.

[92] With the war's ever increasing need for able bodied men consuming America's labor force in the early 1940s, industry turned to teen-aged boys and girls to fill in as replacements.