There were a number of events which propelled Europe into World War in 1914, beginning with a series of entangling (and often overlapping) alliances which were first instituted by Otto von Bismarck at the end of the Franco-Prussian War. Among them:
- The Three Emperors League of 1873 in which Russia, Austria-Hungary and Germany agreed to protect each other against "radical movements," a euphemism for the growing nationalism which was extant in Europe.
- A treaty between Austria and Germany and later Italy to aid each other in the event of Russian attack, known as the Triple Alliance.
- Secret alliances of Germany with Austria and Russia offering each aid and defense should the other attack. This agreement also promised to divide the Ottoman Empire in the event of further conflict there.
- Attempts were made to drive a wedge between France and Britain, but this only succeeded in pushing the two countries together.
This delicate balance may have been maintained by Bismarck, but Kaiser Wilhelm II removed him from office. The actions of Britain and France in allying with each other led many German leaders to believe that Germany was being encircled; and they therefore prepared for war.
In the meantime, nationalism continued to grow in Europe. Peoples living in areas politically designated as a nation other than theirs clamored for national recognition. This was no where more apparent than in the Balkans a portion of which were "administered" by Austria-Hungary. Austria ultimately annexed Bosnia and Herzegovnia, which outraged Serbia, which was helpless.
The indignity suffered by Serbia at Austria's hands led to the assassination of Archduke Ferdinand in Sarajevo on June 28, 1914. Austria then provoked a war with Serbia over the assassination, and called upon its ally Germany. Russia, allied with Serbia, began a mobilization, and Germany declared war on Russia. When Germany did not receive a satisfactory reply from France as to its interests, Germany then declared war on France, and the war was on.
In simplest words, World War I was caused by imperialism, alliances, militarism, and nationalism.