In January 1917, German Foreign Minister Arthur Zimmermann sent a telegram, which came to be known as the "Zimmerman Note," to his minister in Mexico, to inform him that unrestricted submarine warfare would soon resume. If that action were to induce the United States to declare war on Germany, then the German representative in Mexico was authorized to propose an alliance with the Mexicans. As an inducement, Mexico would be offered the opportunity to regain much of the land lost in the Mexican War (New Mexico, Arizona and Texas).
The telegram containing these instructions was intercepted by British Naval Intelligence. However, Prime Minister David Lloyd George decided to withhold the note from the Americans for the time being. Over the next few weeks, the United States drifted closer to entering the war. Germany resumed its submarine offensive on February 1, which prompted the U.S. to break diplomatic relations two days later. American public opinion stewed for several weeks.
On February 24, the British revealed the telegram's contents to American officials. President Wilson realized that the time for equivocation was over and released the text of the note to the press for publication on March 1. The impact on public opinion was immediate and silenced all but the most ardent German supporters. A declaration of war would follow in a little more than a month.