If we make a distinction, as we must, between what caused the first World War and what occasioned it we shall find that most people are content to agree that the immediate occasion was the Sarajevo assassination. Although there can be disagreement even on this issue―perhaps the immediate occasion was the Austrian ultimatum to Serbia, the German rejection of negotiations or the Russian mobilisation?―most people are content not to argue about it. They recognise that the immediate occasion of the war can tell us little of value about the causes of it.

The same is true of the Balkan entanglement which underlay the assassination. Most people accept that this in its turn was only the occasion, though a less immediate occasion, of the general war that followed between the Great Powers―that some crisis, and perhaps some war, was so likely to be generated by the Balkan problem that if the Sarajevo assassination had not sparked off a crisis in July 1914 then some other disturbance in the Balkans would have done so at no distant date. There is general agreement that there was a general war in 1914 because the local strains and pressures set up by a problem with which Europe had lived for nearly a century at last interlocked with a wider condition of tension between the Great Powers of which the origin was much more recent; and that if we are to understand the outbreak of that war, if we seek to know what caused it as opposed to what occasioned it, we must explain this interlocking and the growth of that wider tension.