On 28 July 1914, Austria-Hungary declared war on Serbia.
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.
Here is a newspaper report published the day after Austria-Hungary declared war on Serbia.
Aberdeen Journal – Wednesday 29 July 1914
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