February 24, 1917. In the office of the U.S. Ambassador to the United Kingdom, British Foreign Minister David Balfour hands Ambassador Page a copy of the Zimmerman Telegram. One of modern history's worst diplomatic blunders and greatest successes in signals intelligence has come to light.
It's hard to get over the monumental carelessness that led to the Telegram. See, World War I was straight raging, and America was debating joining the Allies - which obviously stood to tilt the balance against Germany. Arthur Zimmerman, German Foreign Minister, somehow got it into his head that a more regional threat - from, say, Mexico - would keep the USA more concerned with matters close to home and persuade them not to join World War I. Even if they did, a Mexican invasion might keep them off Germany's back long enough for the Reich to win the war.
The Germans had made a practice of giving aid to Mexico as early as 1914, mainly as a half-hearted way to distract America. This was sort of futile, because Mexico was very clearly in no condition to do any invading. Mexico was in the middle of a Revolution, with current populist strongman Venustiano Carranza on the upswing, and it was in miserable shape as a military power. In fact, American troops had occupied Veracruz in 1914 and had launched expeditions into Mexico in 1915 and 1916. If any war between the US and Mexico happened, one country was going to be invaded and it wasn't going to be America.
Nevertheless, Zimmerman seemed to think this was a brilliant idea. He decided to tell Carranza in a secret message (for obvious reasons) that if the United States declared war on Germany, Mexico should attack the United States. In return, Zimmerman promised to give Texas, Arizona and New Mexico back to their original owner.
This was sort of a dumb idea already, but what happened next is truly puzzling in its imbecility. At the beginning of World War I the Royal Navy had cut Germany's transatlantic telegraph cable that linked them with their embassies in the Western Hemisphere. The USA, being a sport, had let Germany use their cables from the American embassy in Berlin to transmit messages to the German embassy in Washington. So Zimmerman decided to send a secret message asking Mexico to attack America...via an American cable.
So that's pretty bad. But at least it was in code. The American ambassador refused to send the coded message at first, but Zimmerman's good-natured manner won him over. So the cable was sent from Berlin via Amsterdam, then London, then Washington, where it was given to the German ambassador and re-transmitted to Mexico City. (I promise all this is important.)
Carranza in Mexico City gathered his generals to discuss Germany's secret offer. They pointed out that if they weren't in the middle of a Revolution, and if the Americans hadn't recently occupied their major seaport, and if their country wasn't poor and starving, and if the United States wasn't overwhelmingly more powerful....then they would still have to invade and occupy a territory several times larger than most European countries. And American citizens, then as now, are very fond of their guns.
So that's a...no. And it probably would have ended there. But instead...
British intelligence services were by several measures the best of the Twentieth Century. By 1916, for one thing, they had cracked the German diplomatic code based on scraps retrieved in Iraq and others forwarded by Russia from a wrecked German ship. They had been very careful in their decisions and public statements to keep Germany from finding out this fact.
And, well, there was the embarrassing fact that they'd been spying on every American transmission that passed through London. Friends spy on friends, fact of life, and the British were extremely interested in American diplomatic traffic. They wanted the USA in the war as much as Germany wanted them out, and so they were trying to persuade their cousins across the pond any way they could.
On January 16, 1917, the British intercepted and decoded the Zimmerman Telegram as it passed through London's cables.
Great! Or...not. This was awkward. The British knew that the Telegram would make the Americans furious at Germany, but there was also the tiny fact that Britain had been hacking American cables. The Americans probably wouldn't like that either. So, frustrated, the British had to sit on this bombshell while they figured out a way to conceal the fact that they'd essentially been peeking into Germany's Gmail on America's laptop when they had permission to use neither.
The British found a way. A British agent in Mexico managed to bribe a telegraph operator for a copy of the telegram, then the intelligence service fabricated a story about multiple transmissions on different lines - which the US would not discover was a lie until many years later.
In February, Germany had once again started up its program of sinking neutral ships with U-Boats, pushing the US towards war. While Congress and the American people hotly debated entering World War I, the British cover story was polished off. Now was the time to release the incriminating Zimmerman Telegram for maximum effect. Best of all, the Telegram contained several gloating references to unrestricted submarine warfare - just as American ships were being sunk across the Atlantic by German U-Boats.
On February 24, British Foreign Minister David Balfour placed the translated Telegram into the hands of American Ambassador Walter Hines Page. Four days later, on February 28, President Woodrow Wilson released it to the press.
It was a big scandal - but not as big as you'd expect. While the Telegram fit in with widespread anti-German and anti-Mexican sentiment, causing some degree of hysteria, the odd British cover story caused many people to dismiss it as an English forgery. War was highly contentious, and it was widely known the Allies were desperate to bring the USA in on their side. Many people thought Britain was trying to trick America into war. The "telegram as forgery" theory was promoted by pacifists, the Hearst newspaper machine, and German diplomats and pro-German activists.
Then, Zimmerman did another dumb thing. Trying to sic Mexico on the USA, like siccing a tired corgi onto a pitbull, was pretty dumb. Sending the message on an American cable was even dumber. But no one had taught German Foreign Minister Arthur Zimmerman the first rule of politics - deny, deny, deny.
On March 3, Arthur Zimmerman announced to the world that the Telegram was genuine. (WHY?!?)
American outrage was immediate. One month later, on April 6, the United States went to war with Germany.
Zimmerman was quietly stripped of all power and resigned five months later.
Idiot.
Book Recommendation: The "classic" history of the Zimmerman Telegram is Barbara W. Tuchman, The Zimmerman Telegram (New York: Viking Press, 1958), but as lovely as Barb's writing always was, she never let facts get in the way of a good story. Try instead Thomas Boghardt, The Zimmerman Telegram: Intelligence, Diplomacy, and America's Entry into World War I (Annapolis, MD: Naval Institute Press, 2012).