Hell_Of_Helios
Heir to the throne of Austria-Hungary, Archduke Franz Ferdinand and his wife Sophie are assassinated during their visit to Sarajevo by a Serbian nationalist named Gavrilo Princip.
Following this, the July Crisis occurs and Austria-Hungary issues an ultimatum to Serbia, demanding humiliating concessions.
It's ignored. And Austria-Hungary declares war.
And so, the first steps towards the 'war to end all wars' are made...