1914

The declaration came as the Austro-Hungarian government declared itself unhappy with the reply given by the Serbian government to an ultimatum issued by Vienna on 23 July.

That ultimatum - presented following the assassination of Archduke Franz Ferdinand - had demanded of the Serbian government a range of measures that no sovereign state could realistically have taken.

That a reply was demanded within two days underlined the sense that Austria-Hungary had already decided on war with Serbia. And when the response that came from Serbia was deemed inadequate, war was indeed inevitable.

Source:

https://www.rte.ie/centuryireland/index.php/articles/austria-hungary-declares-war-on-serbia