Austria declared war on Serbia in a calculated response to the murder of Archduke Franz Ferdinand. There was much blame on both sides as to why war came about and what the true cause was, but the murder was the reason used.
After numerous trials and a handful of executions, the Austrian government still blamed the Serbian government for what happened. Demands were presented to Serbia, giving them 48 hours to meet those demands or go to war. Austria knew that Russia would support the Serbians in their battle, so many in the Austrian government saw it as the first step to putting down many of their rivalries through war. Italy was also on the list of potential targets for the country.