One hundred years ago, a crisis in Mexican-American relations changed the course of history. Front pages blared the news that would precipitate U.S. entry into World War I: the publication of the legendary Zimmermann Telegram. The American people—up to then decidedly isolationist—read the shocking news that Germany was trying to induce Mexico to invade the United States. Public opinion turned on a dime. So before we make an enemy of our southern neighbor, Donald Trump should be briefed on the lessons of March 1, 1917.

By the end of 1916, the Great War had been bogged down in the stalemate of trench warfare for some time at the cost of millions of soldiers’ lives. The German military chieftains were convinced that their only chance at victory was to let loose their feared U-boat submarines on the Atlantic with a mandate to sink any ship, whether enemy or neutral, in an effort to starve England into submission. The great risk in this tactic was possibly pushing the Americans to intervene on the side of the Allies if their own tonnage was sunk.