2. The Zimmermann Telegram. In January 1917, British intelligence intercepted a telegram from German Foreign Secretary Arthur Zimmermann to the German ambassador in Mexico. The telegram stated that Germany would support Mexico in a war against the United States if the US entered the war against Germany. The publication of the Zimmermann Telegram further infuriated the American public and strengthened the calls for war.
3. The declaration of unlimited submarine warfare by Germany. In January 1917, Germany announced that it would resume unrestricted submarine warfare, meaning that it would sink any ship, regardless of nationality, that entered the war zone around the British Isles. This led to the sinking of several American merchant ships and eventually resulted in the US declaration of war on Germany on April 6, 1917.