On 28 of June, 1914, Gavrilo Princip, a member of the "Black Hand" (a group that was calling for a greater Serbia state), assassinated the Austro-Hungarian Empire's Archduke Franz Ferdinand and his wife, in Sarajevo.
The assassination of Franz-Ferdinand and Sophie set off a rapid chain of events: Austria-Hungary, like many in countries around the world, blamed the Serbian government for the attack and hoped to use the incident as justification for settling the question of Slav nationalism once and for all.
As Russia supported Serbia, an Austro-Hungarian declaration of war was delayed until its leaders received assurances from German leader Kaiser Wilhelm that Germany would support their cause in the event of a Russian intervention–which would likely involve Russia’s ally, France, and possibly Britain as well.
On July 28, Austria-Hungary declared war on Serbia, and the tenuous peace between Europe’s great powers collapsed. Within a week, Russia, Belgium, France, Great Britain and Serbia had lined up against Austria-Hungary and Germany, and World War I had begun.