What might have happened if Britain had … But where do you go with that thought? Especially in such grim times as these. If only we had voted against Brexit, perhaps. Or been better prepared for Covid. Gripped the climate crisis more ruthlessly. The list of missed moments and might-have-beens in our recent past is dauntingly long. But always remember this. Might-have-beens are not always more benign options. Missed opportunities can look very different from the Guardian reader-pleasing list above. If only Britain had … Not joined the EU in the first place. Not imposed a Covid lockdown at all. Sent the Windrush generation back. Kept on digging the coal to fire the power stations. Those who see a divine hand in human affairs don’t think much of a question like: “What if events had turned out differently?” Marxists who see history as the working out of the iron laws of dialectical materialism sometimes think the same. EP Thompson, the author of The Making of the English Working Class, once dismissed such speculations as “unhistorical shit”. Thompson was right, gloriously so, about lots of things. But he was wrong about that one. History was the future once. Its formation is always contingent, sometimes on accident, sometimes on conscious choices. As a friend said this week, it’s why diaries are so illuminating for historians – and now for Covid inquiry chairs. Those who write them do not know what comes next. They are driving without lights into the darkness. That is why counterfactual “What if?” exercises are not irrelevant to history. On the contrary. Counterfactuals are more than a game. They can deepen history too. The Dutch historian Johan Huizinga put it this way: “The historian must constantly put himself at a point in the past at which the known factors will seem to permit different outcomes.” So must the newspaper columnist. This summer, Berlin is mounting an absorbing exhibition on this subject. Roads Not Taken, which runs at the German History Museum until November, provides a subtle look at some of the pivotal moments that shaped German history over the past two centuries. You come to each stage of the exhibition knowing what actually happened. But in each case the exhibition also invites you to reflect on what could have happened instead, but did not – the roads not taken. It is a hugely imaginative show. It takes 14 moments that mattered for Germany, starting with the year that the Berlin Wall fell, 1989, and then working backwards in a series of episodes. Finally, it arrives at the revolutionary year of 1848-9, when the Frankfurt parliament, elected by male voters in what were then multiple German states and kingdoms, attempted unsuccessfully to create a unified constitutional monarchy with a charter of fundamental rights. To underline that other outcomes are always possible, the treatment of 1989 is deliberately deflating. Suppose, it asks, East Germans had decided to emulate China’s repression of the Tiananmen Square uprising in the summer of 1989 and had cracked down on their own burgeoning democracy protests. East Germany’s leader Egon Krenz went to Beijing that autumn to congratulate the Chinese leaders. Repression was a genuine option when he came back. It nearly happened. So did the 1970s attempt to overthrow Willy Brandt’s diplomatic detente with West Germany’s eastern neighbours. Back the exhibition goes, through Joseph Stalin’s tantalising proposal of a reunified but neutral Germany in 1952, through the genuine possibility of the first nuclear bomb being dropped not on Hiroshima but on Ludwigshafen in 1945, through the failure of the assassination of Hitler, through the rise of the Nazis, the struggles of the Weimar republic and the failure of the peace movement in 1914. At the end, it asks what might have happened in 1849 if the Frankfurt parliament had actually created the democratic, federal and constitutional Germany that proved to be so elusive over the next century and a half. Historically, that may be the biggest might-have-been of all. Imagine such an exhibition devoted instead to British history. We have not had the same traumatic history that Germany has. But we have had big turning points. What are some of Britain’s roads not taken? One obvious place to start would be Brexit. But we pretty much know what the road not taken would have looked like there. It would have been a continuation of the cautious, conditional and internally besieged Europeanism of the years before 2016. More interesting, perhaps, to imagine what might have happened if Britain had joined the eurozone in 1999. It could have happened. It nearly did. Journeying backwards, where next? Perhaps initially to the Falklands war of 1982 and the all-too-possible sinking of a British flagship that might have stopped the war in the South Atlantic in its tracks and perhaps hastened an end to the country’s habitual military hubris. Or, looking at domestic politics, the still-neglected road not taken after the Bullock report in 1977, which recommended the culture-changing introduction of German-style co-determination between workers and management in British business. A generation before that, what about the Conservative party’s serious internal debate about whether to dismantle the nationalised health service in 1951? Or the very nearly made appointment of Edward Wood as Neville Chamberlain’s successor in 1940 as the Wehrmacht overran France? Or the decision that the pro-Nazi Edward VIII might have taken in 1936 to make a morganatic marriage which might have seen him on the throne at the same time, determined to sue for peace with Hitler? Not all the roads not taken would be sunlit. Delving further back, there would be sections on Ireland and the empire, perhaps in the context of the William Gladstone home rule bills of 1886 and 1893. Sections on Britain’s imperial politics might include Joseph Chamberlain’s attempt to turn back free trade in favour of the empire as a trading bloc, or the early but rare domestic British efforts, by Keir Hardie and others, for Indian independence. The exhibition might end with parliament’s rejection of votes for women in 1867. This merely sketches some options. It would be a formidable exhibition if such a thing ever took place. Yet a British equivalent of the Berlin museum or its thoughtful exhibition – the latest of many it has mounted – remains inconceivable. Unlike Germany, which has had to start over again with its national history, we have never made the generous reset about our past that we need. As a result, our approach to our history remains locked in contemporary politics, partisanship and polarisation. We may have shared these islands with each other for centuries. But we still struggle to share our troubled history. And so we do not know where to go next. Martin Kettle is a Guardian columnist