Austria-Hungary Declares War on Serbia – 28 July 1914
The declaration of war was made after Serbia rejected the terms in an ultimatum sent to them by Austria on July 23, after Archduke Franz Ferdinand, heir to the Austria-Hungary throne, had been assassinated by Serbian student, Gavrilo Princip, in Sarajevo.
The image below is of a newspaper report that was published the day after Austria-Hungary declared war on Serbia.