On April 6, 1917, the United States officially declared war against Germany after the country repeatedly attacked armed and unarmed vessels traveling to Britain, resulting in American casualties. In 1915, Germany had enacted a policy of unrestricted submarine warfare against any ships that entered the British war zone. President Woodrow Wilson maintained neutrality until Germany ignored an agreement to ensure passenger safety before destroying enemy ships.
A major swaying point arose in Jan. 1917, when Britain intercepted the Zimmerman Telegram, a German proposal seeking aid from Mexico in return for recovering ceded territory from the United States.
Despite a pledge to stay out of the war, the U.S. government was a major trading partner of Britain, placing American ships in Germany's line of fire. In early 1915, a German cruiser sunk a private American ship, the William P. Frye, and in May, 128 Americans died when a British ocean liner, the Lusitania, was assaulted while transporting munitions.
In March 1916, German ships sunk the Sussex, an unarmed French vessel, leading President Wilson to demand reparative action from Germany. In the Sussex pledge, Germany agreed to allow all passengers to flee merchant ships under attack before sinking the vessel. However, German naval representatives convinced the government that resuming unrestricted submarine warfare could lead to Britain's defeat within five months. On Feb. 3, 1917, President Wilson officially ended all diplomatic relations with Germany.