If we were still naming wars as colourfully as they used to – the War of the Spanish Succession, the War of Jenkins' Ear – we should call the one that began 100 years ago today the War of Unintended Consequences.

No one, certainly, intended to create what Winston Churchill would later call a "crippled, broken world". Austria-Hungary, which declared war on 28 July 1914, merely wanted to dismember Serbia, where irredentists were stirring up ethnic Serbs in Austrian territory.

Russia, which backed Serbia, wanted to come to the aid of fellow Eastern Orthodox Slavs, and to undo the racial humiliation of losing a war to Japan a decade earlier. Once the fatal tangle of alliances had drawn more countries into the conflict, each one claimed that it was only defending itself against a conspiracy of its enemies. Granted, desire for territory lay beneath the surface – France dreamed of recapturing the lost Alsace-Lorraine, for example, and Germany of establishing its primacy over the tottering tsarist empire in eastern Europe – but these ambitions, too, were limited, not world-changing.

Look, however, at what the war wrought. More than nine million troops were killed, and, depending on how you count them, as many as 10 million civilians. In Turkey, Russia, the Balkans and elsewhere, unprecedented millions of people became homeless refugees. Some 21 million soldiers were wounded. In Britain, 41,000 men had one or more limbs amputated; in France, so many had mangled faces that they formed a National Union of Disfigured Men. The toll was particularly appalling among the young. Of every 20 British men between 18 and 32 in 1914, three were killed and six wounded. Surely many a family shared the feelings of a despairing couple who engraved on their son's tombstone at Gallipoli: "What harm did he do Thee, O Lord?"

Beyond the carnage, the war changed our world for the worse in almost every possible way. Without the vast slaughter, misery and upheaval of the war, would the most extreme group of revolutionaries still have come to power in Russia? And in Germany, the war left a toxic legacy of resentment that Hitler would brilliantly manipulate to win power. Already by 1918, rightwing Germans were blaming the country's military setbacks on the Jews. It is impossible to imagine the second world war without the first.

We will be asked to do a lot of commemorating over these next four and a half years, but whom and what should we commemorate, and in what spirit? Today most people would surely agree that the war of 1914-1918 was not fought for the lofty motives that each side claimed, and that we all would be better off if it had not been fought at all. Before he died, Harry Patch, the last surviving British veteran of the war, said it best: "It was not worth even one life." Yet all the traditional ways we remember wars make little space for this feeling...