The Zimmermann Telegram used American cables (because the German's had been cut) and thus passed through Washington D.C. on its way to Mexico. The Germans weren't supposed to send encrypted messages through this route, but they lied to the U.S. in order to do it. Kind of like using your roommate's Netflix logon info to watch OITNB. And getting caught. (Source)

The German's didn't have a numerical code for the word "Arizona" so it had to be spelled out phonetically (in code) in the Zimmermann Telegram. Imagine how proud they were in Room 40 when they solved that one. High-fives and Guinness all around. (Source)

During World War I, 274 German U-boats (submarines) sank 6,596 ships. That's about 24 ships per sub, because math can be both fun and deadly. (Source)

World War I was called the Great War, the World War, the War of Nations, and most ironically, the War to End All Wars. People didn't start calling it World War I until World War II started. This is an important fact to remember: if you ever go back in time, don't ruin the surprise. (Source)

Barbara Tuchman, in the preface to a later edition of her book The Zimmermann Telegram, wrote that the code used for breaking the telegram was likely found in a codebook wrapped in several pair of long woolen underwear carried by Helmuth Listemann, a German Consul doing business in the Persian Gulf. He was arrested by British staff who were there obviously doing undercover work for L.L.Bean. (Source)