Few would have thought that the assassination of one man would provide the catalyst for the First World War.
In 1914, Archduke Franz Ferdinand, heir to the throne of the Austro-Hungarian Empire, and his wife Sophie, were touring the city of Sarajevo in Bosnia. The date, the 28 June, was known as St Vitus' Day and marked the anniversary of the Battle of Kosova, where in 1389 the Ottoman Turks defeated the Serbs, ending the existence of Serbia as a nation. Hoping to further the cause of Serbian nationalism, Gavrilo Princip a member of the secret society the Black Hand, fired two pistol shots that resulted in the death of Archduke Franz Ferdinand and his wife.
To fully understand the reasons for the First World War we need to review the political climate of Europe in the early 20th century. Serbia and Russia had always had close relations due to their common ethnic heritage. Germany and Austria-Hungary were strong allies, each looking to expand and each with powerful militaries. Serbia was being repressed and persecuted by Austria-Hungary and tensions increased when Austria-Hungary forced Serbia to abandon some of the gains it made during the Balkan wars of 1912-1913. France and Germany were historic enemies, with France had a treaty of alliance with Russia, while England had a treaty with Belgium. .
What existed were two hostile systems of alliance, and this, in reality, was the reason for World War 1. The German and Austro-Hungarian alliance combined armies of such might that it tipped the European balance of power and forced a system of defensive alliances. The powerful Triple Alliance wanted war, all that was needed was an excuse.
The government of Austria-Hungary was obviously upset at the killing. More importantly, they saw the assassination as an endangerment to the Austria-Hungary state. They suspected that the Serbian government had been involved, and that they were encouraging Nationalist movements, making the Austria-Hungary state less stable.