Germanys declaration of unlimited submarine warfare, supplemented by the Zimmerman note proposing an alliance with Mexico, finally caused the United States to declare war. Wilson aroused the country to patriotic heights by making the war an idealistic crusade for democracy and permanent peace based on his Fourteen Points.

Wartime propaganda stirred voluntary commitment to the war effort, but at the cost of suppressing dissent. Voluntary efforts also worked wonders in organizing industry, producing food, and financing the war. Labor, including women, made substantial wartime gains. The beginnings of black migration to northern cities led to racial tensions and riots.

Americas soldiers took nearly a year to arrive in Europe, and they fought in only two major battles at the end of the war. Americas main contribution to the Allied victory was to provide supplies, personnel, and improved morale. Wilsons immense prestige created high expectations for an idealistic peace, but his own political blunders and the stubborn opposition of European statesmen forced him to compromise his lofty aims.

As Lodge stalled the treaty, Wilson tried to rouse the country on behalf of his cherished League, but his own physical collapse and refusal to compromise killed the treaty and the League. Republican isolationists effectively turned Hardings victory in 1920 into a death sentence for the League.