While the United States initially remained neutral, President Wilson was determined to continue the country’s lucrative shipping trade with the European powers. A large portion of trade revenue was derived from shipping ammunition and other war supplies, with the majority of shipments going to the Allied forces of France and Great Britain. In an effort to derail this supply flow, German submarines—widely known as U-boats—sank merchant vessels hauling goods across the Atlantic Ocean.
In May of 1915, a German U-boat struck and sank a British passenger ship from the Cunard Line, the RMS Lusitania. Over 1,000 people lost their lives in the event, among them 128 Americans. The loss of American citizens fanned the flames of nationalism, as recent immigrants expressed loyalty to their new nation rather than to their distant homelands already involved in the conflict. American retaliation was, it appeared, inevitable.