This photograph of Archduke Franz Ferdinand and Duchess Sophie was taken in Sarajevo on June 28, 1914, only moments before their assassination.

The assassination of Archduke Franz Ferdinand by Gavrilo Princip, a member of the Serbian nationalist terrorist group called the Black Hand set into motion a deadly system of alliances. Within a month of the June 28, 1914 assassination, the guns of Europe began to blaze. How did one act of terrorism trigger a war that resulted in roughly 30 million casualties — 20 million wounded and 10 million killed?

The assassination in Sarajevo precipitated WWI, but it did not cause it. The Austro-Hungarian Empire was an anachronism in the 20th century. The forces of nationalism, which celebrated a common ethnic and linguistic heritage, had spread throughout Europe during the 19th century. Nationalists from Serbia, which achieved independence from the Ottoman Empire in 1878, wanted to create a Greater Serbia, combining various groups of South Slavs into a unified country. Within the borders of the Austro-Hungarian Empire lay nearly seven million South Slavs.

Constantly trying to keep up morale, pictures were often taken of the leaders talking to their troops. Here Kaiser Wilhelm II inspects Austrian troops.

Serbian nationalist groups like Union or Death (as the Black Hand was also known) became increasingly active in the years leading up to the assassination, especially after Austria annexed the provinces of Bosnia and Herzegovina in 1908. Further conflict developed with the Balkan War of 1912. Serbia was forced to give up newly acquired territory along the Albanian coast when Russia decided against supporting its claim. Both Serbia and Russia were humiliated in the process by Austria, which refused to allow Serbia to secure an outlet to the sea. The tension between Austria and Serbia increased, as did the Russians' resolve to support their fellow Slavs in Serbia during the next crisis.

Otto von Bismarck created the Triple Alliance with Austria-Hungary and Italy in the 1882 to protect Austria and to prevent Russia from expanding into Eastern Europe. Austrian leaders also knew that its alliance with Germany gave it powerful backing in dealing with Serbia and Russia. Bismarck also engineered a nonaggression treaty with Russia, isolating France. Germany was well positioned. With two major allies to the South and a guarantee of no attack from Russia in the East, Bismarck believed Germany’s position was secure. But Kaiser Wilhelm II forced Bismarck to resign in 1890 and set Germany on a more aggressive path.

Wilhelm first ended the agreement with Russia. Seeking new support, Russia formed a 1904 alliance with France. Meanwhile, the British had become alarmed by Germany's naval build-up, which threatened Britain's colonial empire. In 1907, Britain joined with France and Russia to form the Triple Entente. The six most powerful nations in Europe were now divided into two major camps.

The European countries and empires at the onset of WWI in 1914.

Each side was determined to place itself in a superior position. Militarism in Western Europe became increasingly fashionable, and an arms race soon developed.

The murder of Franz Ferdinand provided the spark that ignited the powderkeg. After the assassination, Austrian leaders vowed to crush Serbian efforts to destroy their empire, and to bring the assassin of their heir-apparent to justice. On July 23, they issued an ultimatum to Serbia to comply with a series of demands. Serbia agreed to their terms, with one exception — officials refused to allow Austria to enter Serb territory to investigate the assassination. Looking for a pretext to wage war, Austria seized on this act of resistance and on July 28 it declared war on Serbia, setting into motion the alliance system.

Within days, Russia, France, Germany, and Britain had mobilized their armies. Russia was an historic ally of Serbia because of a similar Slavic heritage. On July 30, Russia mobilized its armies in support of Serbia, triggering a German declaration of war on August 1.