Serbian campaign

Following the assassination of Archduke Franz Ferdinand on 28 June 1914, Austria-Hungary attacked Serbia. Russia was bound by treaty to Serbia, and began to mobilise troops against Austria-Hungary. Germany was allied to Austria-Hungary, and so declared war against Russia. France had a treaty with Russia, so entered the war against Germany, which invaded Belgium, allied with France. Britain, at the time also allied with France, therefore declared war against Germany. This brought in Britain's colonies including Australia, New Zealand, Canada, South Africa and India. Japan, which had an agreement with Britain, also entered the war. (PROSE: The Time Traveller's Almanac)