World War I (WWI), which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918. It involved all the world's great powers.
At the outbreak of the war the United States pursued a policy of non-intervention, avoiding conflict while trying to broker a peace. When a German U-boat sank the British liner Lusitania in 1915, with 128 Americans aboard, U.S. PresidentWoodrow Wilson claimed that "America is too proud to fight" but demanded an end to attacks on passenger ships. Germany complied. Wilson unsuccessfully tried to mediate a settlement. However, he also repeatedly warned that the U.S.A. would not tolerate unrestricted submarine warfare, in violation of international law and U.S. ideas of human rights. Wilson was under pressure from former presidentTheodore Roosevelt, who denounced German acts as "piracy". Wilson's desire to have a seat at negotiations at war's end to advance the League of Nations also played a role in the eventual decision to join the war. Wilson's Secretary of State, William Jennings Bryan, whose opinions had been ignored, resigned in 1915, as he could no longer support the president's policy. Public opinion was angered at suspected German sabotage of Black Tom in Jersey City, New Jersey, and the Kingsland Explosion.
In January 1917, Germany resumed unrestricted submarine warfare. The German Foreign Minister, in the Zimmermann Telegram, told Mexico that U.S. entry was likely once unrestricted submarine warfare began, and invited Mexico to join the war as Germany's ally against the United States. In return, the Germans would send Mexico money and help it recover the territories of Texas, New Mexico, and Arizona that Mexico had lost during the Mexican-American War 70 years earlier. Wilson released the Zimmerman note to the public, and Americans saw it as casus belli— a cause for war.