Zimmermann Telegram

On January 16, 1917, Arthur Zimmermann, Germany's foreign secretary, sent a coded telegram to the German minister in Mexico. It said Germany was about to resume unrestricted submarine warfare, which risked bringing the United States into World War I. If that happened, the minister was to propose an alliance: Mexico would attack the United States with German money and support, and in return recover Texas, New Mexico and Arizona, lost in the war of 1846 to 1848. Mexico was also to invite Japan to switch sides.¹²

Germany's own transatlantic cables had been cut by Britain at the start of the war, so the message traveled partly over routes that passed through British stations, including a cable the Americans had allowed Germany to use for diplomatic traffic. British naval intelligence in the Admiralty's Room 40 intercepted and partly decoded it. Its chiefs then faced a dilemma: revealing the telegram might expose that Britain was reading both German and American cables. They obtained a second copy from the Mexico City telegraph office, which let them claim it had been stolen in Mexico.¹²³

Britain handed the telegram to the U.S. ambassador on February 24, and President Woodrow Wilson released it to the press on March 1. Many Americans first suspected a British forgery, but on March 3 Zimmermann publicly admitted he had sent it. Mexico's president Venustiano Carranza judged the plan unrealistic and declined. Together with German submarines sinking American ships, the telegram swung opinion, especially in the West and Southwest, and Congress declared war on April 6, 1917. Historians still weigh how much the telegram itself mattered compared with the U-boat campaign.¹³