On June 28 1914 Archduke Franz Ferdinand and his wife, who were the heir to the throne in Austria-Hungary, were assassinated by a member of the Black Hand, Garvrilo Princip. The country's reaction was to stamp its authority upon the Serbian government by issuing an ultimatum which demanded that the assassins be brought to justice. Russia announced mobilisation of an army to defend Serbia. Germany was allied to Austria-Hungary and thought the mobilisation was an act of war against the country. France was bound to Russia by a treaty and soon found itself war against Germany. As a result Germany invaded Belgium who was neutral so that they could reach Paris as quickly as possible. Britain was loosely allied to France by a treaty which placed a moral obligation upon her to defend France.
Britain then declared war against Germany but since Germany invaded Belgium, Britain decided to defend Belgium. Britain was also at war with Austria-Hungary while defending Belgium against Germany. Australia, Canada, India, New Zealand and Union of South Africa provided military assistance for Britain.
Japan had an agreement with Britain and declared war on Germany on 23 August 1914. Austria-Hungary seeing that Japan was dealing with Germany decided to invade Japan. Although Italy was allied to Germany and Austria-Hungary, the country avoided entering the war, only to defend both countries in 'defensive' war.
