Christopher Clark writes:
The European continent was at peace on the morning of Sunday, 28 June 1914, when Archduke Franz Ferdinand, the Austrian heir to the throne, and his wife, Sophie Chotek, arrived at Sarajevo railway station. Thirty-seven days later, it was at war. In its complexity and the speed with which it escalated, the ‘July Crisis’ of 1914 is without parallel in world history. Franz Ferdinand and his wife were assassinated that morning in Sarajevo by Bosnian Serb students acting for a shadowy Belgrade-based ultranationalist network. The Austrian government in Vienna resolved to serve an ultimatum on its Serbian neighbour. Berlin promised support for Austria on 5 July. Encouraged by Paris, Russia opted to defend its Serbian client by mobilising against Austria and Germany. Unsatisfied by the Serbian reply to its ultimatum, Austria declared war on Serbia. Germany mobilised against France and Russia. France asked London for help. On 4 August 1914, following the German breach of Belgian neutrality, Britain entered the war.