Authored by Edward Luce, originally posted at FT.com,

It was tempting to believe history had turned a page. Alas, posterity may see Barack Obama’s 2008 election as a detour from the path an angry America took after 9/11. Mr Obama called for an open dialogue with the Muslim world. Donald Trump and his team have all but declared war on Islam. Mr Obama believed there was no problem that could not be salved by reason. Mr Trump has opposite instincts. Whatever precise form Mr Trump’s administration takes, we know this: Mr Obama’s legacy will be purged. In many cases all it will take is the stroke of Mr Trump’s pen.

The Obama erasure will go far deeper than undoing domestic laws, or foreign deals. Mr Trump will repeal Obamacare, or alter it beyond recognition. He will “keep an open mind” about whether to pull the US out of the Paris agreement on climate change and quite probably blow up the US-Iran nuclear deal. These acts would undo Mr Obama’s most visible achievements. Less obvious ones, such as the ban on Arctic drilling and enhanced interrogation techniques and the intention of closing Guantánamo Bay (never completed) will also be consigned to the dustbin. It will be as if Mr Obama was never here.

The bizarre thing is that the same America which elected Mr Trump is already missing Mr Obama. This often happens to outgoing presidents. But in Mr Obama’s case it is unusually sharp. At 55 per cent his job approval is equivalent to Ronald Reagan’s at the same point, and ahead of Bill Clinton’s. It is more than 20 points higher than George W Bush’s. The more Mr Trump tweets from the gut, the more Americans appreciate Mr Obama’s calm weighing of pros and cons. “No drama Obama” is even making the case that Mr Trump be given a chance to succeed. “I think nothing is the end of the world until it is the end of the world,” he told the New Yorker.

Yet it is the end of the world as Mr Obama knew it. He must take some responsibility for its demise. He took office at a moment when the US-led global order was in the balance. Mr Bush’s ill-planned wars in Iraq and Afghanistan had shredded America’s standing in the Middle East and beyond. The financial crisis of 2008 had brought the US-created “Washington consensus” of free market orthodoxy into possibly terminal disrepute. America’s power was in decline but it was not too late to do something about it.

Mr Obama took office at a geopolitical inflection point. As he prepares to leave, few any longer dispute the fact of relative US decline. For all his high aspirations, Mr Obama was unable to stop the process. Can Mr Trump reverse it? One of Mr Obama’s core traits is to believe that reason governs how people act. It is the perennial failing of liberal technocrats to suppose human affairs are settled by rational argument. When people failed to see the merits of the case — whether Republican legislators, or foreign leaders — Mr Obama would retreat into injured silence. The world has been a disappointment to Mr Obama. When Vladimir Putin’s Russia annexed Crimea in 2014, John Kerry, the outgoing secretary of state, said: “You just don’t in the 21st century behave in a 19th-century fashion by invading another country on [a] completely trumped up pretext.” But that is how the world often operates. The US had done just that to Iraq in the 21st century.

America tends to choose presidents whose personalities are the opposite of their predecessors. Mr Obama, the reasoner, replaced Bush the decider. Mr Trump is a pure bluffer. Much like one of his property deals, Mr Trump will cajole, bully, flatter, and bribe whomever he is dealing with. When he fails — which he surely must — he will declare it a success and divert people’s attention. That is what his outrageous tweets are about. When people tell the truth about him, he will call them liars. When they praise him, he will call them geniuses. When a crisis strikes, he will gamble from the gut. Buyers’ remorse will grip the US public. Mr Obama’s post-presidential approval ratings are likely to stay up.

But Mr Trump will not reverse America’s relative decline. The chances are he will drastically accelerate it. The global role that Mr Obama inherited — and tried, to some degree, to uphold — is now in tatters. It would be hard to overstate the epochal significance of Mr Trump’s election. The US-led international order as we knew it for 70 years is over. The era of great power politics is back. An ebullient Russia, led by the strongman Putin, and an increasingly confident China, led by the strongman Xi Jinping, will deal with a wounded America led by strongman Trump. The long-term trajectory is towards China. But the short-term drama will focus on Mr Trump’s dealings with Mr Putin. How they play out is anybody’s guess. But it will not be pretty. Europe will be the loser. So too will American prestige.

Mr Obama, meanwhile, will settle down to his memoirs. If the past is any guide, they will be beautifully written. Here was a highly intelligent leader, and a fundamentally decent one, who strived to make the case for international co-operation to a world that was not really listening. Mr Obama came bearing hope. He leaves warning against fear. But the world’s attention has wandered. People are highly fearful — and rightly so.