Assassination of Archduke Franz Ferdinand and his wife Sophia at the hands of Bosnian Serb Gavrilo Princip.
The first World War was set into motion with an assassination. On June 28, 1914, a Serbian nationalist group, known as the Black Hand, ambushed and killed the inspector general of the Austria-Hungary army, Archduke Franz Ferdinand. In response, tensions rose amongst European nations. Several countries formally declared sides with either the Allies (Russia, England and France) or the Central Powers (Germany, Austria-Hungary, the Ottoman Empire and Bulgaria.)
Gavrilo Princip, only 19 at the time of his arrest, would die less than 4 years into his life prison sentence.
The Zimmerman Telegram
What finally drove the U.S. to declare war was an encrypted telegram. In February 1917, British intelligence gave the decoded message to President Wilson. The German Foreign Minister, Arthur Zimmermann, had sent the telegram to Mexico. It said if Mexico joined Germany, Mexico would be granted back the territories it had recently lost to America (Texas, New Mexico and Arizona). When published in U.S. papers, this spread anti-German sentiment and pro-war emotions across the U.S.