"World War I
Austria-Hungary issues ultimatum to Serbia
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. As the German ambassador to Vienna reported to his government on July 14, the [note] to Serbia is being composed so that the possibility of its being accepted is practically excluded.
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." - History.com