Despite the U.S. position, many Americans personally sympathized with Britain, France, and their allies. American institutions lent large sums to the Allied governments, giving the U.S. a financial stake in the outcome of the war. Nearly 10% of Americans identified as ethnic Germans, most of whom hoped the United States would remain neutral in the war.
In November of 1916, President Woodrow Wilson won a close re-election under the slogan “He Kept Us Out of War.” Yet in early 1917 when Russia’s internal political revolutions effectively took them out of the war against Germany, the prospects for the Allies darkened. Already receiving massive shipments of supplies and a near limitless line of credit from the U.S., the Allies needed reinforcements.
When easing Eastern military pressures made more forces available for their Western Front, Germany sensed the tide was turning. To capitalize on the shift, German leaders agreed in January of 1917 to resume unrestricted submarine warfare to break the devastating army stalemate in Europe and the British navy’s successful blockade of critical German supply ports. This pushed American public opinion toward intervention... Germany’s unrestricted submarine warfare strategy sent more merchant and passenger ships to the ocean’s floor and the loss of American lives mounted. The U.S. protested and in February severed diplomatic relations with Germany, while Congress appropriated funds for increased military affairs.About the same time, British cryptographers intercepted and began deciphering Germany’s “Zimmermann Telegram” offering U.S. territory to Mexico in return for joining the German cause. Though Mexico declaring war was not perceived as an imminent threat by the American public, sensational headlines trumpeted each new development as one of history’s most influential acts of codebreaking played out. Across the nation, support grew for intervention.
On March 20, almost a month after the Zimmerman Telegram hit the American press, President Wilson convened the Cabinet to discuss moving from a policy of armed neutrality to war. It was unanimous: all members advised war. With a proclamation already being drafted by President Wilson, the American steamship Aztec was torpedoed and sunk by Germany on April 1.On April 2, President Wilson asked Congress to declare war against Germany specifically citing Germany’s renewed submarine policy as “a war against mankind. It is a war against all nations.” He also spoke about German spying inside the U.S. and the treachery of the Zimmermann Telegram. Wilson urged that “the world must be made safe for democracy.”