Serbia was competing with the Austro-Hungarian Empire for influence over Bosnia. Each of the two powers wanted Bosnia in its camp to ensure dominance in the region. Serbia dreamed of collecting several of the Balkan states into a southern Slavic kingdom. Austria already had Hungary, (Czech) Bohemia, and related territories; Bosnia would have sealed Austrian preeminence in southeastern Europe. The archduke was the heir apparent, and a Serbian assassination would have aptly expressed Balkan rage at perceived Austrian arrogance.
Upon the death of his son, Emperor Franz Joseph issued a series of demands and ultimatums to the Serbian government. Given the already-tense relations between the two countries, the emperor's demands - which were severe and attached to short timelines - were rejected by Serbia. This brought Serbia to the brink of war with Austria-Hungary.
By any measure, Serbia was smaller and less powerful than Austria. It was willing to risk this lopsided confrontation because it was allied with Russia, and counted on the Tsar's support. Thus Russia was brought into the situation.
Austria, for its part, was counting on its alliance with Germany. If war were declared, Emperor Wilhelm II would support Emperor Franz Joseph. It is clear, however, that none of these parties really wanted war. They also did not want to be perceiving as backing away from confrontation - they were afraid that this would be perceived as cowardice.
What was originally a dispute between Austria and Serbia became a second-hand quarrel between Germany and Russia. Frantic, last-minute communications between Emperor Wilhelm II and Tsar Nicholas II reveal that both sides still wanted to avoid war, but again neither was willing to show any conciliatory behavior.
Matters deteriorated still further when Russian military experts decided that its army could not be partially mobilized: it was an all-or-nothing proposition. The decision for full mobilization sent the wrong signal to Germany and Austria. War was by then inevitable.
This narrative reveals one cause of the war: a system of alliances and treaties, some public and some secret. What might have been a regional war between two powers turned into a World War.
Other causes were present: militarism, which despised diplomatic attempts at resolution and preferred to rely on threats, and which fostered an atmosphere in which a nation's military was the preeminent measure and symbol of a nation's strength; nationalism, which, although subject to varying definitions and stages, is often a value system which places the existence and growth of the nation-state as the ultimate value both for the individual and for the society as a whole, surpassing pacifying values like family and religious faith; and industrialism, which saw nations and armies as markets for military technology, and saw wars as opportunities for selling millions of rifles and uniforms.
France and England were brought into the war by further ripple-effects through the network of alliances and treaties. America, which entered the war much later, can safely be said to have had no hand in its cause.
In whichever ratio, these potential causes had one thing in common: they all created an atmosphere of competition rather than cooperation.
