On February 1, 1917, Germany announced that it would begin unrestricted submarine warfare against all vessels entering or leaving British waters and shipping lanes around Britain itself. This meant that any ship could be sunk without warning by German submarines as far away as 600 miles from land. It also meant that Americans traveling on those ships were at risk of being torpedoed by German U-boats if they were traveling in those waters. Many Americans were angry at Germany’s policy of unrestricted submarine warfare because it was costing them money (the British navy stopped allowing food shipments into Europe so they could feed their own people).
On January 31st, 1917, after months of secret negotiations with Mexico’s president Venustiano Carranza y Pino Suárez over their respective claims to territory lost during the Mexican-American War (1846–48), German Foreign Minister Arthur Zimmermann sent a coded telegram titled “Zimmermann Telegram”. It was a coded message sent from German Foreign Minister Arthur Zimmermann to his ambassador in Mexico City. The German government wanted to encourage Mexico to declare war on the United States so it would be forced to fight on two fronts instead of one. In return for joining Germany in its war against the United States, Mexico would be given back Texas, New Mexico, and Arizona.
The telegram was intercepted by British intelligence agents who passed it on to American intelligence officials. President Woodrow Wilson released it to Congress on March 1, 1917, and asked for a declaration of war against Germany two days later.