Why did the United States break diplomatic relations with Germany in 1917?
The Zimmerman Telegram
A message from the German Foreign Minister to Mexico's Ambassador to Germany offered Mexico a shot at getting Texas, New Mexico and Arizona (a major part of the lands lost in the Mexican War some 80 years earlier) back if they allied with Germany, in the event America entered the war (which they had successfully avoided doing for three years).
Named for the Foreign Minister who sent it, it was called the Zimmerman Telegram. It played into then-recent resentment of American military incursions into Mexican territory (Pershing pursuing Pancho Villa) and it asked Mexico to inveigle Japan into the war on Germany's behalf.
Mexican authorities considered the offer, but decided it was not practical. Excluding the prospect of taking on the US Army, they didn't want to face a civilian population that was better armed than the Mexican army.