With American trade becoming more and more lopsided toward the Allied cause, many feared that it was only a matter of time before the United States would be at war. The issue that propelled most American fencesitters to side with the British was German submarine warfare.
The British, with the world's largest navy, had effectively shut down German maritime trade. Because there was no hope of catching the British in numbers of ships, the Germans felt that the submarine was their only key to survival. One "U-boat" could surreptitiously sink many battleships, only to slip away unseen. This practice would stop only if the British would lift their blockade.
The isolationist American public had little concern if the British and Germans tangled on the high seas. The incident that changed everything was the sinking of the Lusitania. The Germans felt they had done their part to warn Americans about the danger of overseas travel.
The German government purchased advertisement space in American newspapers warning that Americans who traveled on ships carrying war contraband risked submarine attack. When the Lusitania departed New York, the Germans believed the massive passenger ship was loaded with munitions in its cargo hold. On May 7, 1915, a German U-boat torpedoed the ship without warning, sending 1,198 passengers, including 128 Americans, to an icy grave. The Lusitania, as it turned out, was carrying over 4 million rounds of ammunition.
President Wilson was enraged. The British were breaking the rules, but the Germans were causing deaths.
Wilson's Secretary of State, William Jennings Bryan, recommended a ban on American travel on any ships of nations at war. Wilson preferred a tougher line against the German Kaiser. He demanded an immediate end to submarine warfare, prompting Bryan to resign in protest. The Germans began a 2-year practice of pledging to cease submarine attacks, reneging on that pledge, and issuing it again under U.S. protest.
Wilson had other reasons for leaning toward the Allied side. He greatly admired the British government, and democracy in any form was preferable to German authoritarianism. The historical ties with Britain seemed to draw the United States closer to that side.
Many Americans felt a debt to France for their help in the American Revolution. Several hundred volunteers, appropriately named the Lafayette Escadrilles, already volunteered to fight with the French in 1916. In November of that year, Wilson campaigned for re-election with a peace platform. "He kept us out of war," read his campaign signs, and Americans narrowly returned him to the White House. But peace was not to be.
In February 1917, citing the unbalanced U.S. trade with the Allies, Germany announced a policy of unrestricted submarine warfare. All vessels spotted in the war zone would be sunk immediately and without warning. Wilson responded by severing diplomatic relations with the German government.
Later that month, British intelligence intercepted the notorious Zimmermann telegram. The German foreign minister sent a message courting support from Mexico in the event the United States should enter the war. Zimmermann promised Mexico a return of Texas, New Mexico, and Arizona — territories it had lost in 1848.
Relations between the U.S. and Mexico were already strained. The U.S. had sent troops across the border in search of Pancho Villa, who had conducted several cross-border raids of American towns. Failing to find Villa, the troops had been withdrawn only in January 1917. Despite the recent souring between Mexico and its Northern neighbor, the United States, the Mexican government declined the offer. In a calculated move, Wilson released the captured telegram to the American press.
A tempest of outrage followed. More and more Americans began to label Germany as the true villain in the war. When German subs sank several American commercial ships in March, Wilson had an even stronger hand to play. On April 2, 1917, he addressed the Congress, citing a long list of grievances against Germany. Four days later, by a wide margin in each house, Congress declared war on Germany, and the U.S. was plunged into the bloodiest battle in history.
Still, the debate lived on. Two Senators and fifty Representatives voted against the war resolution, including the first female ever to sit in Congress, Jeannette Rankin of Montana. Although a clear majority of Americans now supported the war effort, there were large segments of the populace who still needed convincing.