At six o’clock in the evening on July 23, 1914, nearly one month after the assassination of Austrian Archduke Franz Ferdinand and his wife by a young Serbian nationalist in Sarajevo, Bosnia, Baron Giesl von Gieslingen, ambassador of the Austro-Hungarian Empire to Serbia, delivers 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.

According to the terms of the ultimatum delivered on July 23, the Serbian government would have to accept an Austro-Hungarian inquiry into the assassination, notwithstanding its claim that it was already conducting its own internal investigation. Serbia was also to suppress all anti-Austrian propaganda and to take steps to root out and eliminate terrorist organizations within its borders—one such organization, the Black Hand, was believed to have aided and abetted the archduke’s killer, Gavrilo Princip, and his cohorts, providing weapons and safe passage from Belgrade to Sarajevo. The Dual Monarchy demanded an answer to the note within 48 hours—by that time, however, anticipating Serbian defiance, Gieslingen had already packed his bags and prepared to leave the embassy.

While the world waited for Serbia’s response, Germany worked diplomatically to contain the effects of the ultimatum, but none of the other great powers, with reason, were inclined to see Austria-Hungary, with its relatively weak military, as acting alone. By 1914, the battle lines had been drawn in Europe: if Germany stood with Austria-Hungary against Serbia (and by extension, Russia) then Russia’s allies, France and Britain, would be likely to step into the fray as well.

The British cabinet, just after receiving the news of the Austrian note to Serbia, held a meeting in London, one that had previously been devoted to discussing Ireland’s desire for independence. This note, as Winston Churchill famously wrote, was clearly an ultimatum, but it was an ultimatum such as had never been penned in modern times. As the reading proceeded it seemed absolutely impossible that any State in the world could accept it, or that any acceptance, however abject, would satisfy the aggressor. The parishes of Fermanagh and Tyrone faded back into the mists and squalls of Ireland, and a strange light beganto fall upon the map of Europe.

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.

Meanwhile, in Belgrade on the afternoon of July 25, convinced that Austria-Hungary was preparing for a fight, Serbian Prime Minister Nicola Pasic ordered the Serbian army to mobilize. Pasic himself delivered the Serbian answer to the ultimatum to Gieslingen at the Austrian embassy, just before the 6 p.m. deadline. Serbia’s response effectively accepted all terms of the ultimatum but one: it would not accept Austria-Hungary’s participation in any internal inquiry, stating that this would be a violation of the Constitution and of the law of criminal procedure. This response did much to appeal Pasic and his country to international observers of the conflict; to Vienna, however, it made little difference. Gieslingen, bags packed and car waiting to drive him to the railroad station, broke the Dual Monarchy’s diplomatic relations with Serbia and left to catch his train. Three days later, on July 28, 1914, Austria-Hungary declared war on Serbia, beginning the First World War.

READ MORE: World War I: Causes and Timeline