The causes of the First World War can be divided into two categories: the immediate causes and the underlying causes.
The immediate cause of the First World War was the assassination of Franz Ferdinand, the heir to the Austro-Hungarian throne, by members of the Black Hand, a Serbian terrorist organization. Austria-Hungary reacted to the assassination with a severe ultimatum to Serbia that Serbia could not accept without seriously compromising its sovereignty. When Serbia only accepted nine of the ten demands, Austria-Hungary declared war on Serbia on July 28. Within a week, the main European powers (Russia, Germany, France, and Great Britain), bound by various treaties of defence, entered the war. On July 30 Russia began mobilization against Austria-Hungary. Germany then declared war on Russia on August 1 and, after France began mobilizing, on France on August 3. After Germany violated Belgium's neutrality, Great Britain declared war on Germany on August 4.
Even before the assassination of Franz Ferdinand, however, Europe was like a dry tinder-box awaiting a spark. There are several underlying causes that made war highly likely. Here are some of the main ones:
- Already alluded to was the web of alliances and secret treaties that made it likely that a war involving one of the major European powers would lead to other powers becoming involved.
- Increased militarism: Since the 1870s, all of the major powers had been preparing for war, resulting in an ever-increasing arms race. The military expenses of most of the European powers had grown by a factor of three or more since the 1870s.
- The Balkans were a particular trouble-spot in Europe. They had been the scene of frequent conflict, including wars in 1912 and 1913. They were home to many different ethnic groups with nationalist aspirations. The Great Powers also had aspirations in the Balkans.