In many ways the First World War created the modern world. It set the stage for the Second World War and was the crucible for the Russian Revolution and the Cold War that succeeded it. There were two short-term causes of WW1: the system of interlocking alliances between the European powers and the assassination of the heir to the Austro-Hungarian throne.
Europe before the war
Europe in 1914 was a continent of empires rather than countries. The vast, multicultural Austro-Hungarian Empire, ruled by the Hapsburg dynasty, straddled Central Europe. Eastern Europe was part of the Russian Empire, ruled by the Tsars. Britain still presided over an empire that covered one-fifth of the earth’s surface, and France, Belgium and Spain had imperial possessions overseas as well. The empires lived in fear of each other, and so developed a series of interlocking alliances meant to ensure that no one country could attack any other with impunity.
In the years before WW1, two blocs of alliances developed. The wily German Chancellor Otto von Bismarck created one bloc when he negotiated an alliance with both the Austro-Hungarian Empire and the Russian Empire. In 1892 Russia fell out of this pact and created a rival alliance with France. In 1904 France and Britain became allies (the Entente Cordiale), and Britain aligned with Russia in 1907. These interlocking agreements cemented the second bloc of Britain, France and Russia, known as the Triple Entente. With Russia now on the other side, Germany was faced with the possibility of a war on two fronts.
Tensions in Europe built up, on both the local and international levels.. On the international level, each of the blocs was vying for prominence, and an arms race raged between Britain and Germany. On the local level, within the Austro-Hungarian Empire, some ethnic minorities sought to separate from the rule of the Hapsburg emperors. Nowhere were the tensions sharper than in the volatile Balkans region (again the scene of inter-ethnic fighting almost a century later, in the 1990s).
On 28 June 1914, the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand of Austria, was visiting the city of Sarajevo in Bosnia, an area recently annexed to Austro-Hungary. Members of the Serbian minority in Bosnia, who sought unification with the independent state of Serbia, backed by a faction of the Serbian army, plotted the murder of the Archduke. Nineteen-year-old Gavrilo Princip, a member of the underground Bosnian Serb nationalists, shot and killed the Archduke and his wife, Sophie.
The slide into war
Austro-Hungary accused Serbia of being behind the murder. Along with its ally Germany, Austro-Hungary threatened Serbia, seeking a limited war which would absorb the Balkan state. In response, Serbia’s ally Russia mobilized her army in a show of force against Austro-Hungary. Then Germany mobilized, but knew it would have to crush Russia’s ally, France, before turning east. On the 2nd of August, Germany invaded Belgium on its way to attack France and also declared war on Russia. The same day France declared war on Germany, and two days later, on the 4th of August 1914, Britain declared war on Germany in support of France. Without anyone actually wanting it, the world was at war.