I have just finished reading 'Sleepwalkers - How Europe went to war in 1914' by Christopher Clark, Professor of Modern History in Cambridge. He describes the statesmen who stumbled into War in 1914 as “sleepwalkers, watchful but unseeing, haunted by dreams, yet blind to the horror they were about to bring to the world”.
A web of interlocking commitments, designed to give individual countries security and peace behind their own borders, ended up tumbling the whole continent into War.
Austro-Hungary had a defensive pact with Germany. Russia set itself up as the protector of Serbia. France gave Russia a blank cheque in the Balkans because it needed Russian assurances against Germany. Britain had a rather more vague understanding with France. It feared any Russian rapprochement with Germany because Russia could threaten British interests in India.
So, when Franz Ferdinand was murdered in Sarajevo by assassins that had come from Serbia, the possibility that all these dominoes might fall in the direction of war opened up. But it was only a possibility.
Serbia could have taken resolute action to root out the conspiracy behind the assassins before Austria issued any ultimatum. Austria could have issued a more temperate ultimatum. Serbia could have given a less evasive response. Germany could have restrained Austria.