German efforts to use their submarines ("U-boats") to blockade Britain resulted in the deaths of American travelers and sailors, and attacks on passenger liners caused public outrage. Most notable was torpedoing without warning the passenger liner "Lusitania" in 1915. Germany promised not to repeat; however it reversed position in early 1917, believing that unrestricted U-boat warfare against all ships headed to Britain would win the war, albeit at the cost of American entry. When Americans read the text of the German offer to Mexico, known as the Zimmermann Telegram, they saw an offer for Mexico to go to war with Germany against the United States, with German funding, with the promise of the return of the lost territories of Arizona, New Mexico, and Texas. On Apr 1, 1917, Wilson called for war, emphasizing that the U.S. had to fight to maintain its honor and to have a decisive voice in shaping the new postwar world. Congress voted on April 6, 1917 to declare war, but it was far from unanimous.