Although most of the American public was sympathetic to the Allies, the United States stayed neutral for nearly three years. However, in early 1917, a series of events finally drew America into the war.
On February 1, Germany resumed unrestricted sub- marine warfare. Britain’s naval blockade was causing severe shortages for Germany’s military and its civilian economy. Germany saw no reason to continue holding back its most effective naval weapon, especially considering how unprepared the U.S. was for war. German U-boats once again began sinking all ships travelling to or from Allied ports. In response, the United States broke off diplomatic relations with Germany.
Then, in mid-February, the British presented the American government with a telegram they had inter- cepted from Germany’s Foreign Minister Arthur Zim- merman to its Ambassador to Mexico. The message promised to help Mexico recapture territory it had lost to the United States in the Mexican-American War (1846-48) if Mexico joined Germany as an ally. The Zimmermann Telegram (as it came to be called) was published in U.S. newspapers on March 1, further feeding U.S. outrage.
In March, the Germans sank five U.S. merchant ships, with the loss of dozens of American lives. In addition, the fall of the Tsar’s regime in Russia simplified the moral argument for the Allied cause: the war was now between democratic nations and autocratic empires.
On April 2, President Wilson went to the U.S. Congress to formally ask it to declare war on Germany. The Senate granted Wilson’s request two days later, followed by the House of Representatives on April 6. The United States had joined the war.