Four days after Austria-Hungary declared war on Serbia, Germany and Russia declare war against each other, France orders a general mobilization, and the first German army units cross into Luxembourg in preparation for the German invasion of France. During the next three days, Russia, France, Belgium, and Great Britain all lined up against Austria-Hungary and Germany, and the German army invaded Belgium. The “Great War” that ensued was one of unprecedented destruction and loss of life, resulting in the deaths of some 20 million soldiers and civilians.
On June 28, 1914, in an event that is widely regarded as sparking the outbreak of World War I, Archduke Franz Ferdinand, heir to the Austro-Hungarian empire, was shot to death with his wife by Bosnian Serb Gavrilo Princip in Sarajevo, Bosnia. Ferdinand had been inspecting his uncle’s imperial armed forces in Bosnia and Herzegovina, despite the threat of Serbian nationalists who wanted these Austro-Hungarian possessions to join newly independent Serbia. Austria-Hungary blamed the Serbian government for the attack and hoped to use the incident as justification for settling the problem of Slavic nationalism once and for all. However, as Russia supported Serbia, an Austria-Hungary declaration of war was delayed until its leaders received assurances from German leader Kaiser Wilhelm II that Germany would support their cause in the event of a Russian intervention.
On July 28, Austria-Hungary declared war on Serbia, and the tenuous peace between Europe’s great powers collapsed. On July 29, Austro-Hungarian forces began to shell the Serbian capital of Belgrade, and Russia, Serbia’s ally, ordered a troop mobilization against Austria-Hungary. France, allied with Russia, began to mobilize on August 1. France and Germany declared war against each other on August 3.
After crossing through neutral Luxembourg, the German army invaded Belgium on the night of August 3-4, prompting Great Britain, Belgium’s ally, to declare war against Germany.