On June 28th, 1914, Franz Ferdinand, the heir to the Austro-Hungarian throne, was assassinated by Gavrilo Princip, a nationalist working for the Serbian underground organisation "Black Hand" (officially known as "Unification or Death"). From today's perspective, the assassination is seen as the event that triggered the chain reaction leading to the outbreak of the Great War. These events are known today as the July Crisis. Only one month after the shot, on July 28th, Austria-Hungary declared war on Serbia.
