On February 3 President Woodrow Wilson suspended diplomatic relations between the United States and Germany. German U-boats, meanwhile, continued to fire on Allied merchant vessels, sinking 781,500 tons during the month of February alone. The President had just won a very close election by making a virtue of his great forbearance toward the German aggressors, and he now found his cabinet almost at blows over whether or not to join the war. While he was considering his options, the British Secret Service released to him a decoded telegram from Germany’s foreign minister, Arthur von Zimmerman, to its ambassador to Mexico. The extraordinary document promised “unrestricted submarine warfare” while endeavoring “to keep the United States neutral.” Should that fail, Germany proposed an alliance with Mexico against the United States, with an understanding that in exchange “Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona.”

Wilson was infuriated, not only by the cable’s content but by the way it had been transmitted. As a neutral government, the United States had allowed Zimmerman to send his message over its own State Department wires. The White House did not make the telegram public until March 1, but three days before that Wilson requested permission from Congress to arm merchant vessels against unrestricted submarine attack. The disclosure of the telegram insured passage of the resulting Armed Ship bill in the House, 403 to 14—and complicated things for the group opposing the bill in the Senate.