Before there were surveillance cameras on every corner, governments tapping cell phones, and WikiLeaks revealing state secrets, there was Room 40.
You can tell that Room 40 is all kinds of cool just based on the fact that it has such a vague name. (Shmoop, anyone?)
Room 40 was the name used by a group of code breakers who worked within the British Navy during World War I. As you might have guessed, they sometimes used room number 40 in the British Admiralty Building to do their work—although it wasn't an all-the-time sort of thing. Other people sometimes needed the room too and it's rude to Bogart it. Just because the name of the room is the same as the name of your little club doesn't mean you get to use it forever. Have some respect for your coworkers and learn how to share.
Where were we?
Oh yeah, the guys in room 40 decrypted the Zimmermann Telegram among lots of other messages that the Germans were sending (secret messages, they incorrectly assumed) during the war.
Two really hard-core daredevil things happened that allowed Room 40 to decipher Zimmermann's message to Mexico.
First, the Brits were on the ball. They wasted no time in interrupting Germany's communications. Just hours after war was declared, the British Navy went out into the Atlantic and cut the telegram cables connecting Germany to much of the rest of the world.
Transatlantic cables are literally super long cables running through the oceans connecting continents to each other so that people can send electronic messages back and forth. But not for the Germans, because British sailors hauled theirs out of the Atlantic and sawed it in half.
Without transatlantic telegram cables of their very own the Germans spent the entirety of World War I asking to use the telegram cables of other countries that they hadn't yet invaded or alienated with all their ruthless killing. Like that super annoying friend who's always losing her cell phone and then asking to borrow yours, only she's actually using it to send messages to her other friends about how much she hates you. The German's options were understandably limited, making it easy for Room 40 to intercept German telegrams pretty much whenever they wanted.
In fact, the cables that the Zimmermann telegram used were American. President Wilson was so desperately clinging to his dream of not going to war that he was like, "Sure, use our cables. You guys are negotiating peace, right?" and the Germans were like, "Uh…yeah, peace, that's right," when what they were really doing was talking smack about he U.S. using the U.S. telegram service.
The second thing that Room 40 had that allowed them to figure out what Zimmermann was up to was stolen German codebooks. For years, the British had been hoarding books and clues so that when German ships and U-boats talked to each other in uber-secret navy-talk, the Brits knew what they were saying.
So, when the Zimmermann telegram was sent, Room 40 intercepted, decoded, and translated it—and then stood around it with their mouths hanging open at its totally unbelievable contents.
Almost immediately, officials in the British government wanted to use the Zimmermann Telegram to get the Americans into the war against Germany. But they couldn't just come right out and admit that they were spying on German telegrams, because then the Germans would stop sending them, or start using new encryptions. So instead, the British pretended that they had a mole working in the German embassy in Mexico (where the telegram was sent) and that he stole the already decrypted Zimmermann Telegram for them.
The plan worked and had the added bonus of sending Heinrich von Eckardt, the German ambassador to Mexico, into a frenzy trying to figure out who the mole was. Because of the Zimmermann Telegram, the U.S. joined the war on the Allied side— exactly what the Brits wanted and the Germans didn't.
It wasn't quite as edge-of-your seat as a good spy thriller, but it was certainly a sly piece of espionage with dramatic consequences.