After the Archduke Franz Ferdinand of Austria and his wife were killed by a Serbian nationalist in Sarajevo on 28 June 1914, Austria-Hungary, supported by Germany, presented Serbia with an ultimatum on July 23, 1914, demanding, among other things, that all anti-Austrian propaganda within Serbia be suppressed, and that Austria-Hungary be allowed to conduct its own investigation into the archduke’s killing.
Though Serbia effectively accepted all of Austria’s demands except for one, the Austrian government broke diplomatic relations with Serbia on July 25 and went ahead with military preparedness measures.
Meanwhile Russia—Serbia’s own mighty supporter in the Balkans—began its own steps towards military mobilization against Austria.
Note: I am undertaking this tour through the centenary events of World War I over the commemoration dates as a personal learning experience and my own small tribute to the people who lost their lives and/or their loved ones in the always terrible acts of war. If people who are knowledgeable about these events care to correct, expand and educate me, I will be perfectly content. I apologise to those who have landed up here while on a scholarly quest. This is NOT intended as an academic commentary.