The Zimmerman Telegram in 1917

By its third year, World War I had taken millions of lives in Europe and throughout the world. However, with the entire European continent engulfed in conflict, the United States adopted a policy of neutrality because it was unwilling to commit its resources or personnel to a foreign war. Even so, it maintained trade with the Allies (France, Russia, and the United Kingdom), supporting them with arms and food exports. War-time trade with Allied nations occurred to the great advantage of U.S. industries. Between 1914 and 1916, U.S. trade with the Allies tripled to $3 billion annually. However, as U.S. ships overwhelmingly composed Allied supply lines, they became a target. The German navy adopted a policy of unrestricted submarine warfare. Germany used U-boats to sink any ships found approaching British waters, including neutral vessels. This policy was widely considered a violation of international law and resulted in the deaths of hundreds of Americans. Most notably, 128 Americans were killed in the 1915 sinking of the passenger liner Lusitania at the hands of German submarines. As a result of international condemnation, Germany halted attacks on U.S. ships after this incident. However, the carnage left the American public enraged. The debate over the merits of neutrality grew louder.

On one hand, neutrality allowed the United States to trade while reserving its own resources and keeping personnel out of harm’s way. Entry into the war would require a massive mobilization of both people and industries. Moreover, the United States lacked the military resources and personnel needed to commit ground troops on the European continent. Some Americans saw the war in Europe as too remote to pose sufficient threat to the United States to justify the costs of war. Though they found the shipping attacks egregious, many observers doubted that Germany could launch a significant attack on the U.S. mainland. However, there were some  Americans who contended that Germany’s aggression could not be ignored. After the sinking of the Lusitania, many policymakers argued that German attacks constituted violations of American neutrality and necessitated war. Other supporters of U.S. involvement saw the war as offering opportunities to promote democracy worldwide and prevent future wars. U.S. businesses and banks also had a vested interest in the outcome of the conflict. Financial institutions in the United States loaned vast sums to the Allies and risked enormous loss if the Allies were defeated. 

With the war expanding, policymakers began to wonder how much longer neutrality would remain an option. Growing tensions with Mexico, also a neutral country, underscored this danger. Sporadic border clashes and persistent U.S. meddling in the Mexican revolution left the neighbors with a mistrustful relationship. An ocean separated the United States from the war in Europe, but U.S. policymakers feared the possibility that Mexico could ally with Germany and the other Central Powers to bring the war to U.S. soil. The United States feared that the Central Powers could try to leverage Mexico to tie down American troops and equipment that otherwise would have been sent to Europe. Moreover, the Central Powers could use Mexico to trigger a conflict directly threatening the U.S. mainland.