Wilson also repeatedly warned that America would not tolerate unrestricted submarine warfare, as it was in violation to American ideas of human rights. Wilson was under great pressure from former president Theodore Roosevelt, who denounced German "piracy" and Wilson's cowardice.
In January 1917, Germany announced it would destroy all ships heading to Britain. Although Wilson broke off diplomatic ties with Germany, he still hoped to avert war by arming merchant vessels as a deterrent. Nevertheless, Germany began sinking American ships immediately.
In February 1917, British intelligence gave the United States government a decoded telegram from Germany's foreign minister, Arthur Zimmerman that had been intercepted en route to his ambassador to Mexico.
The Zimmerman Telegram The Zimmerman Telegram authorized the ambassador to offer Mexico the portions of the Southwest it had lost to the United States in the 1840s if it joined the Central Powers. However, because Wilson had run for re-election in 1916 on a very popular promise to keep the United States out of the European war, he had to handle the telegram very carefully.
Wilson did not publicize it at first, only releasing the message to the press in March after weeks of German attacks on American ships had turned public sentiment toward joining the Allies. On 2 April 1917, Wilson asked Congress for a declaration of war and four days later all but six senators and fifty representatives voted for a war resolution. The Selective Service Act that was passed the following month, along with an extraordinary number of volunteers, built up the army from less than 250,000 to four million over the course of the conflict. General John Pershing was appointed head of the American Expeditionary Force (AEF) and led the first troops to France On April 6th 1917, America declared war on Germany
These were the incidents that precipitated war - but these specific incidents underlined the larger strategic picture: that Germany had zero respect for the USA and was prepared to act with extreme aggression toward American interests without a second thought. This is why Wilson wanted to do anything he could to avoid confronting a victorious Germany down the road. He could theoretically have kowtowed to Germany instead.