On February 1, 1917, the lethal threat of the German U-boat submarine raises its head again, as Germany returns to the policy of unrestricted submarine warfare it had previously suspended in response to pressure from the United States and other neutral countries.

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 attacks on merchant ships followed, culminating in the sinking of the British ship Lusitania by a German U-boat on May 7, 1915. Although the Lusitania was a British ship and it was carrying a supply of munitions—Germany used these two facts to justify the attack—it was principally a passenger ship, and the 1,201 people who drowned in its sinking included 128 Americans. The incident prompted U.S. President Woodrow Wilson to send a strongly worded note to the German government demanding an end to German attacks against unarmed merchant ships. By September 1915, the German government had imposed such strict constraints on the operation of the nation’s submarines that the German navy was persuaded to suspend U-boat warfare altogether.

German navy commanders, however, were ultimately not prepared to accept this degree of passivity, and continued to push for a more aggressive use of the submarine, convincing first the army and eventually the government, most importantly Kaiser Wilhelm, that the U-boat was an essential component of German war strategy. Planning to remain on the defensive on the Western Front in 1917, the supreme army command endorsed the navy’s opinion that unrestricted U-boat warfare against the British at sea could result in a German victory by the fall of 1917. In a joint audience with the kaiser on January 8, 1917, army and naval leaders presented their arguments to Wilhelm, who supported them in spite of the opposition of the German chancellor, Theobald von Bethmann Hollweg, who was not at the meeting. Though he feared antagonizing the U.S., Bethmann Hollweg accepted the kaiser’s decision, pressured as he was by the armed forces and the hungry and frustrated German public, which was angered by the continuing Allied naval blockade and which supported aggressive action towards Germany’s enemies.

On January 31, 1917, Bethmann Hollweg went before the German Reichstag government and made the announcement that unrestricted submarine warfare would resume the next day, February 1: "The destructive designs of our opponents cannot be expressed more strongly. We have been challenged to fight to the end. We accept the challenge. We stake everything, and we shall be victorious."