Some history and some great propaganda posters;
To the American public of 1914, the outbreak of war in Europe came as a shock. At first the encounter seemed remote, but its economic and political effects were swift and deep. By 1915 U.S. industry, which had been mildly depressed, was prospering again with munitions orders from the Western Allies. Both sides used propaganda to arouse the public passions of Americans -- a third of whom were foreign-born or had one or two foreign-born parents. Moreover, Britain and Germany both acted against U.S. shipping on the high seas, bringing sharp protests from President Woodrow Wilson. But the disputes between the United States and Germany grew increasingly ominous.
In February 1915, German military leaders announced that they would attack all merchant shipping on the waters around the British Isles. President Wilson warned that the United States would not forsake its traditional right, as a neutral, to trade on the high seas -- a view of neutral rights not shared by Germany or Great Britain. Wilson declared that the nation would hold Germany to "strict accountability" for the loss of American vessels or lives. Soon afterward, in the spring of 1915, when the British liner Lusitania was sunk with nearly 1,200 people aboard, 128 of them Americans, indignation reached a fever pitch.
Anxious to avoid a possible declaration of war by the United States, Germany issued orders to its submarine commanders to give warning to ocean-going vessels -- even if they flew the enemy flag -- before firing on them. But on August 19, these orders were ignored and the British steamer Arabic was sunk without warning. In March 1916, the Germans torpedoed the French ship Sussex, injuring several Americans. President Wilson issued an ultimatum stating that unless Germany abandoned its present methods of submarine warfare, the United States would sever relations. Germany agreed.
As a result, Wilson was able to win reelection that year, partly on the strength of his party's slogan: "He kept us out of war." As late as January 1917, in a speech before the Senate, Wilson called for a "peace without victory," which, he said, was the only kind of peace that could last.
On January 22, 1917, the German government gave notice that unrestricted submarine warfare would be resumed. When five U.S. vessels had been sunk by April, Wilson asked Congress for a declaration of war. Immediately, the government set about mobilizing its military resources, industry, labor and agriculture. By October 1918, on the eve of Allied victory, a U.S. army of over 1,750,000 soldiers had been deployed in France.
The U.S. Navy was crucial in helping the British break the submarine blockade, and in the summer of 1918, during a long-awaited German offensive, fresh American troops, under the command of General John J. Pershing, played a decisive role on land. In November, for example, American forces took an important part in the vast Meuse-Argonne offensive, which cracked Germany's vaunted Hindenburg Line.
President Wilson contributed greatly to an early end to the war by defining the war aims of the Allies, and by insisting that the struggle was being waged not against the German people but against their autocratic government. His famous Fourteen Points, submitted to the Senate in January 1918 as the basis for a just peace, called for abandonment of secret international agreements, a guarantee of freedom of the seas, the removal of tariff barriers between nations, reductions in national armaments, and an adjustment of colonial claims with due regard to the interests of the inhabitants affected. Other points sought to ensure self-rule and unhampered economic development for European nationalities. The Fourteenth Point constituted the keystone of Wilson's arch of peace -- the formation of an association of nations to afford "mutual guarantees of political independence and territorial integrity to great and small states alike."
By the summer of 1918, when Germany's armies were being beaten back, the German government appealed to Wilson to negotiate on the basis of the Fourteen Points. The president conferred with the Allies, who acceded to the German proposal. An armistice was concluded on November 11.
http://odur.let.rug.nl/~usa/H/1994/ch9_p2.htm