“America is back,” said President Joe Biden earlier this year, and the entire democratic world breathed a sigh of relief. But as we watch the debacle of the United States’ withdrawal from Afghanistan – Kabul as Saigon 2 – a ghostly voice whispers to us: what if America is not back? What if it is never coming back? What happens then? The Chinese century? Europe as new leader of the free world? Or just plain old international anarchy? If only this were like Saigon in 1975. The US humiliation in Vietnam, following Watergate, marked a low point for America’s reputation in the world. But within a decade, the US was back. By 1995, it seemed to be bestriding the globe as an unchallenged hyperpower. Everyone knows that this time is different. The United States’ self-inflicted domestic problems are 10 times more profound and structural than they were in the mid-1970s – partly because, following the pattern of over-extended empires throughout history, it has spent trillions of dollars in places such as Afghanistan and Iraq, rather than doing more nation-building at home. Abroad, it faces not a declining Leninist-ruled superpower, the Soviet Union, but a rising Leninist-ruled superpower, China. Climate change is the only hyperpower now. Realistically, the most that can be expected is that the United States “comes back” as the leading western power on the international stage, first among equals in a global network of democracies. Resentment of bad American behaviour in various parts of the world, up to and including this latest tragedy, should not close the eyes of any small-d democrat anywhere to the fact that all likely alternatives to that scenario are worse. And the Biden administration is the best chance we have of getting there. Imagine that the pathetic, corrupt, US-funded Afghan government and its supposedly 300,000-strong US-trained army had held out for a few weeks longer, and there had been an orderly withdrawal, with no Chinooks chuntering over an embassy flat roof – that single image more powerful than a million words. There would have been much unhappiness, to be sure, and a sense of failure. But we could have reflected that the experienced US president was soberly executing a hard-nosed plan to bring America back internationally, in a strategic posture better suited to face the 3C challenge his administration has identified: Covid, climate, China. It is still possible that the US can resume that course. Events, like objects, always look larger from close up. Those scenes at Kabul airport will never be forgotten, but with time will fade into a new perspective. Nonetheless, this is a moment to contemplate the alternative: that the US never makes it “back” to a position of international leadership. What then? China almost certainly becomes a dominant power in Asia, but not the predominant one. Japan, India and Australia, not to mention a United States still present in the Indo-Pacific, will all work to avoid that. In China itself, the contradictions between an increasingly Leninist political system, with power concentrated in the hands not just of one party but of one man, and a complex, developed capitalist economy and society, will, sooner or later, deliver its own internal crisis. The search for enhanced nationalist legitimacy by adventures abroad might be the immediate result: watch out, Taiwan. But this is not a formula for a “Chinese century” in the way that one could to some extent plausibly talk about an American century – or at least an American 20 years, between 1989 and 2009 – and, before that, a British century. China already exerts great influence in some European countries, but it is not going to become the leading power in Europe. Neither is Russia, although Vladimir Putin, like Xi Jinping, will doubtless be gloating over this latest American setback. So here’s a more optimistic scenario, one to make French hearts sing. How about Europe stepping into the breach? The EU becomes leader of the free world! Reversing the famous dictum of a former British foreign secretary, George Canning, we summon the old world to redress the balance of the new. As an English European, I would love to see something like this happen. In fact, with friends from all over our continent, I have spent a good deal of time working in a networked organisation called the European Council on Foreign Relations precisely to promote a more coherent, effective European external policy. But it’s not looking good. One European leader, French president Emmanuel Macron, has the vision but not the means. The likely German successors to Chancellor Angela Merkel, following this September’s election, have the means but not the vision. Britain has just left the club, only for leading British Conservatives now to complain loudly about the US leaving us in the lurch over Afghanistan. This is hardly a recipe for global leadership. That leaves a third global alternative: international anarchy. Competing great powers, tribes and interests. A “G-Zero” world, as the geopolitical analyst Ian Bremmer has dubbed it. At worst, a kind of Afghanistan writ large. Quite apart from the misery this would bring to millions of human beings, it risks frying the planet. This summer’s apocalyptic wildfires on Greek islands and floods in Germany, not to mention the climate scientists’ recent authoritative warnings, make it plain that we need an unprecedented level of global collective action to address the climate crisis. Yet the geopolitical moment is one that makes such global collective action more difficult to achieve. I recently watched an American science-fiction film called Arrival, in which vaguely octopus-like aliens land at locations in the US, Russia, China, Saudi Arabia and other key world powers. It turns out their purpose is to get the warring tribes of humankind to cooperate because “we will need you in 3,500 years’ time”. And yes, since this is an American film, they succeed. Absent the intervention of clairvoyant intergalactic octopuses, however, it’s up to us. Seen on a larger canvas, this geopolitical moment demands the active commitment of Europe and China, India, Japan, Australia and many others. And it still requires the US to come back to a leading role in the concert of democracies, just no longer a hegemon. A development economist once brutally observed that there was only one thing worse for poor countries than being exploited, and that was not being exploited. Not just for the west but for an imperilled world, there is one thing worse than US leadership and that is the lack of it. Timothy Garton Ash is a Guardian columnist