How did U-boats and a coded telegram bring America into World War I?

U-boats and a coded telegram pushed a neutral United States toward war because together they made Germany look like a threat to American lives and to American soil. Germany's resumption of unrestricted submarine warfare in February 1917, together with the Zimmermann telegram, brought American entry into the war on the British side.

Start with the U-boats. Unrestricted submarine warfare means sinking merchant ships such as freighters and tankers without warning. Customary cruiser rules said that civilian and neutral ships could be interfered with only if carrying contraband, and that the lives of the crew should be protected. In 1915 the German navy tried a first campaign; it did not significantly affect Britain's goods traffic but took a heavy civilian toll. In May 1915 Lusitania was sunk in a few minutes, killing over a hundred American passengers. Facing US anger, Germany excluded passenger vessels from attack and publicly suspended unrestricted warfare after the sinking of Arabic in August 1915.

By early 1917 Admiral Holtzendorff argued successfully for resuming unrestricted attacks at a greater scale, hoping to starve Britain into surrender. The German high command knew this meant war with the United States but calculated that American mobilization would be too slow to stop a German victory on the Western Front.

Then came the telegram. On 17 January 1917 the German Foreign Office proposed, in secret, a military contract between Germany and Mexico if the United States entered the war, with Mexico to recover Texas, Arizona and New Mexico. British codebreakers in Room 40 read it. When it was made public, many Americans first suspected a forgery, until the German foreign secretary, Arthur Zimmermann, confirmed it was true on 3 March 1917. On 6 April 1917 Congress voted to declare war on Germany.