Europe's imperial powers had been engaged in economic and military rivalries for decades, but these became increasingly virulent during the first part of the 20th century. Great Britain's dominance of the seas came under aggressive challenge from Germany. Brooding over a defeat at the hands of Prussia in 1871, France aligned itself with Great Britain, its traditional enemy, against Germany. Britain, France, and Russia formed an alliance that pressured Germany on both its east and west borders. Germany, in tum, aligned with the ancient kingdom of Austria-Hungary and with the Turkish-ruled Ottoman Empire to its south. The world's great nations were in position for a fight, a single incident away from catastrophe.
On June 28, 1914, the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand, was murdered - along with his wife - while touring the city of Sarajevo in the Austro-Hungarian province of Bosnia-Herzegovina. The assassin, Gavrilo Princip, acted on behalf of Serbian nationalists who sought to expel the Austrians from the Balkans and to create a larger Serbian state.
The assassination did not immediately lead to hostilities, but because Serbia was aligned with Russia, larger alliances were called into play and tensions were immediately heightened. When Serbia refused to investigate links between the assassins and members of the Serbian government, Austria-Hungary declared war on the Serbs in late July. Russia began to mobilize its enormous army to defend its ally, Serbia; Germany felt threatened and declared war on Russia soon after and then, on August 3, it went to war with France as well. The German army crossed into neutral Belgium on its way to France, prompting Britain to enter the war on France's side.