HOW World War I Started: Crash Course World History 209
Most historians agree that the event that started World War I was the assassination of
Austrian archduke Franz Ferdinand on June 28th, 1914
WWI
A war fought from 1914 to 1918 between the Allies, notably Britain, France, Russia, and Italy, and the Central Powers: Germany, Austria-Hungary, Bulgaria, and the Ottoman Empire.
Franz Ferdinand's uncle didn't particularly like him, but as the emperor of Austria-Hungary, he felt a certain responsibility to, do something. Now it's a bit of schoolyard bully kind of diplomacy, but it does make a certain sense,
if Serbia can get bigger, then all of the other places will think that they can have have nations too. Soon enough, you don't have an empire. Almost a month after the assassination, on July 23, Austria issued an ultimatum to Serbia. And Austria intentionally made the demands so harsh that the Serbs would inevitably have to reject them and ergo war.
Looking at why a war or any historical event happened means looking for a cause and effect relationship that
implicitly assumes that if one particular event in a chain of events had gone differently, the historical outcome would also be different.
Black Hand
the Serbian terrorist group that planned to assassinate Franz Ferdinand