People who see a divine hand or the iron laws of dialectical materialism at work in human affairs bridle at the question: "What if things had turned out differently?" To EH Carr, historian of Soviet Russia, to speak of what might have happened in history, as opposed to what did happen, was just a "parlour game". To EP Thompson, author of The Making of the English Working Class, such counterfactual speculation was "unhistorical shit".
Other historians have confessed to being more intrigued. "The historian must constantly put himself at a point in the past at which the known factors will seem to permit different outcomes," wrote Johan Huizinga. It is important to recognise that, at any moment in history, there are real alternatives, argued Hugh Trevor-Roper.
Happily, none of this argument deters the writers of fiction or the public. Germany's possible defeat of Britain in 1940 is by some distance the national treasure trove of might-have-beens. As long ago as 1964, the film It Happened Here by Kevin Brownlow and Andrew Mollo raised the then unthinkable thought that collaboration would have thrived in Hitler's Britain. More recently, a succession of novels, including Robert Harris's Fatherland, Resistance by Owen Sheers and CJ Sansom's Dominion – which imagines a Vichy Britain in 1952 ruled by Lord Beaverbrook and Oswald Mosley – have explored the same theme.
By comparison, the first world war has been the subject of far less counterfactual speculation. Niall Ferguson is one of the exceptions, in an essay (Virtual History. Alternatives and Counterfactuals) which considers the possibility that Britain might have stood aside from the European war in August 1914. And although his essay suffers from the fact that the Eurosceptic Ferguson is over-eager to portray the kaiser as the godfather of the later European Union, his account of the cabinet debates of 1914 is fascinating because Herbert Asquith's Liberal government could so easily have decided to stay out of the war – and very nearly did.
With the centenary of the first world war almost upon us, 2014 is likely to witness plenty of debate about the right forms of commemoration and about whether the war achieved anything. At present, argument about the war mainly consists of two mutually uncomprehending camps. On the one hand, there are those who, as Margaret MacMillan put it recently, think the war was "an unmitigated catastrophe in a sea of mud". On the other, there are those who insist that it was nevertheless "about something". At the time, says MacMillan, people on all sides thought they had a just cause. "It is condescending and wrong to think they were hoodwinked."
But what was the something that the first world war was about? To answer that it was a war between empires, which it surely was, is fine as long as some effort is made to distinguish between the empires. But this rarely happens in a debate that is polarised between collective myths of national sacrifice on the one hand (certainly in Britain and France) and an indiscriminate muddy catastrophe on the other.
The more one tries to examine and maybe get beyond these dominant narratives, as we should next year and as the centenary rolls on, the more a bit of the counterfactual may help the process.
The first world war came to an end in November 1918, when the German armies surrendered near Compiegne. But it could plausibly have ended in a very different way in spring 1918, if Ludendorff's offensive on Paris and towards the Channel had succeeded. It nearly did so. And what might 20th-century Europe have been like if it had?
Obviously, it would have been dominated and shaped by Germany. But what kind of Germany? The militaristic, conservative, repressive Prussian power created by Bismarck? Or the Germany with the largest labour movement in early 20th-century Europe? German history after 1918 would have been a contest between the two – and no one can say which would have won in the end.
But one can say that a victorious Germany, imposing peace on the defeated allies at the treaty of Potsdam, would not have had the reparations and grievances that were actually inflicted upon it by France at Versailles. As a consequence, the rise of Hitler would have been much less likely. In that case, neither the Holocaust nor the second world war would necessarily have followed. If Germany's Jews had survived, Zionism might not have had the international moral force that it rightly claimed after Hitler's defeat. The modern history of the Middle East would therefore be very different – partly also because Turkey would have been among the victors in 1918.
In the kaiser's Europe, defeated France would be the more likely seedbed for fascism, not Germany. But with its steel and coal still in German-controlled Alsace-Lorraine, France's military and naval potential would have been contained. Meanwhile, defeated Britain would have seen its navy sunk in the Heligoland Bight, have been forced to cede its oil interests in the Middle East and the Gulf to Germany, and have been unable to contain Indian nationalism. In practice, the British empire would have been unsustainable. Today's Britain might have ended up as a modest north European social democratic republic – like Denmark without a prince.
Meanwhile America, whose entry into the war would have been successfully pre-empted by Germany's victory, would have become a firmly isolationist power and not the enforcer of international order. Franklin Roosevelt would solve America's postwar economic problems in the 1930s, but he would never fight a war in Europe – though he might have to fight one against Japan. The Soviet Union, with a wary but powerful neighbour in victorious Germany, would have been the great destabilising factor but it might not have been invaded as it was in 1941. And with no second world war there might never have been a cold war either.
A parlour game? Obviously. But at least we can see that the outcome mattered. Europe would have been different if Germany had won in 1918. It would have been grim, repressive and unpredictable in many ways. But there is a plausible case for saying many fewer people would have died in 20th-century Europe. If nothing else, that is worth some reflection. The first world war was a catastrophe in the mud. But it was about something more than tragic sacrifice too. The outcome – what happened and what did not – made a difference. In 2014 we need to get beyond the rival national perspectives and learn to see the war more objectively and thoughtfully than has yet happened.
Martin Kettle is an associate editor of the Guardian and writes on British, European and American politics, as well as the media, law and music.