Austro-Hungarian entry into World War I

On 28 July 1914, Austria-Hungary declared war on Serbia because of the Assassination of Archduke Franz Ferdinand. Within days, long-standing mobilization plans went into effect to initiate invasions or guard against them and Russia, France and Britain stood arrayed against Austria and Germany in what at the time was called the "Great War", and was later named "World War I" or the "First World War".

Source: Wikipedia — Austro-Hungarian entry into World War I (CC BY-SA 4.0)