Great article on Cryptography during World War One that explains how the Zimmerman telegram was encrypted: https://www.probabilisticworld.com/cryptography-during-world-war-i/
And another good article that explains how the British intercepted the Zimmerman Telegram (by stealing it from the U.S., decoding it, and returning it- kind of): https://medium.com/lapsed-historian/breaking-the-zimmermann-telegram-b34ed1d73614
5-1 Causes of World War One: http://www.historyonthenet.com/WW1/causes.htm
5-2 Assassination of Archduke Ferdinand: http://www.eyewitnesstohistory.com/duke.htm
5-3 The Sinking of the Lusitania: http://www.eyewitnesstohistory.com/lusitania.htm
5-4 The Battle of Verdun: http://www.firstworldwar.com/battles/verdun.htm
5-5 The Battle of the Somme: http://www.firstworldwar.com/battles/somme.htm
5-6 The Russian Revolution: http://www.local-life.com/st-petersburg/articles/1917-russian-revolution
5-7 The U.S. in World War One:
http://www.ushistory.org/us/45.asp
http://www.ushistory.org/us/45a.asp
5-8 The German Peace Offensive: http://www.fsmitha.com/h2/ch08b.htm
5-9 Military Reversal Against Germany: http://www.fsmitha.com/h2/ch08b-2.htm