At six o’clock in the evening on July 23, 1914, nearly one month after the assassination of Franz Ferdinand and his wife by a young Serbian nationalist in Sarajevo, Bosnia, the ambassador of the Austro-Hungarian Empire to Serbia, delivered an ultimatum to the Serbian foreign ministry.

Acting with the full support of its allies in Berlin, Austria-Hungary had determined in the aftermath of Franz Ferdinand’s assassination to pursue a hard-line policy towards Serbia. Their plan, developed in coordination with the German foreign office, was to force a military conflict that would, Vienna hoped, end quickly and decisively with a crushing Austrian victory before the rest of Europe—namely, Serbia’s powerful ally, Russia—had time to react.

On receipt of the ultimatum, Serbia at once appealed to Russia, whose council of ministers met on July 24 to determine a course of action. Russian Foreign Minister Sergey Sazonov voiced his belief that Germany was using the crisis over the archduke’s death as a pretext for starting a preventive war to defend its interests in the region. Defying Austro-German expectations that Russia would back down in the case of such a conflict, the council agreed to order four military districts to prepare for mobilization.

On July 28, 1914, Austria-Hungary declared war on Serbia, beginning the First World War.