In the summer of 1914, tensions across the continent of Europe were nearing a breaking point. The fuse that ignited "The Great War" (World War I) was the assassination of the Archduke Ferdinand and his wife, Duchess Sophie -- heirs to the Austro-Hungarian throne in Vienna -- in Sarajevo by a Bosnian Serb student named Gavrilo Princip.
The Archduke had avoided an earlier attempt on his life that morning by blocking a hand-thrown bomb with his hand (it detonated under his car, wounding 20 along the crowded streets of Sarajevo). After tersely scolding the Mayor of Sarajevo about "getting bombs thrown at [him]", he continued with his planned speech -- after which the Duchess suggested they travel to the hospital to visit the wounded citizens.
A wrong turn put the open-air car right in front of another one of the six plotters. Gavrilo Princip pushed his way to the car and shot the Archduke and Duchess with his 9x17mm semi-automatic pistol.
The subsequent reactions by the "great powers" ignited long-dormant animosities. Compounded by the arms race (particularly in naval affairs, with the British royal family competing with their close relative in Germany, the Kaiser Wilhelm II) and a fragile balance of power, the subsequent conflict would result in 20 million deaths and sow the seeds for Hitler's rise to power.