Unrestricted submarine warfare was first introduced in World War I in early 1915, when Germany declared the area around the British Isles a war zone, in which all merchant ships, including those from neutral countries, would be attacked by the German navy. A string of German attacks on merchant ships—culminating in the sinking of the British passenger ship Lusitania on May 7, 1915—led President Wilson to put pressure on the Germans to curb their navy. Fearful of antagonizing the Americans, the German government agreed to put restrictions on the submarine policy going forward, incurring the anger and frustration of many naval leaders, including the naval commander in chief, Admiral Alfred von Tirpitz, who resigned in March 1916.
On March 24, 1916, soon after Tirpitz's resignation, a German U-boat submarine attacked the French passenger steamer Sussex, in the English Channel, thinking it was a British ship equipped to lay explosive mines. Although the ship did not sink, 50 people were killed, and many more injured, including several Americans. On April 19, in an address to the U.S. Congress, President Wilson took a firm stance, stating that unless the Imperial German Government should now immediately declare and effect an abandonment of its present methods of warfare against passenger and freight carrying vessels this Government can have no choice but to sever diplomatic relations with the Government of the German Empire altogether.
To follow up on Wilson's speech, the U.S. ambassador to Germany, James W. Gerard, spoke directly to Kaiser Wilhelm on May 1 at the German army headquarters at Charleville in eastern France. After Gerard protested the continued German submarine attacks on merchant ships, the kaiser in turn denounced the American government's compliance with the Allied naval blockade of Germany, in place since late 1914. Germany could not risk American entry into the war against them, however, and when Gerard urged the kaiser to provide assurances of a change in the submarine policy, the latter agreed.
On May 6, the German government signed the so-called Sussex Pledge, promising to stop the indiscriminate sinking of non-military ships. According to the pledge, merchant ships would be searched, and sunk only if they were found to be carrying contraband materials. Furthermore, no ship would be sunk before safe passage had been provided for the ship's crew and its passengers. Gerard was skeptical, writing in a letter to the U.S. State Department that German leaders, forced by public opinion, and by the von Tirpitz and Conservative parties would take up ruthless submarine warfare again, possibly in the autumn, but at any rate about February or March, 1917.
Gerard's words proved accurate, as on February 1, 1917, Germany announced the resumption of unrestricted submarine warfare. Two days later, Wilson announced a break in diplomatic relations with the German government, and on April 6, 1917, the United States formally entered World War I on the side of the Allies.