Much of the time, argues David Runciman, a British academic, politics matters little to most people. Then, suddenly, it matters all too much. Donald Trump's term as America's 45th president, which is due to begin with the inauguration on January 20th, stands to be one of those moments.

It is extraordinary how little American voters and the world at large feel they know about what Mr Trump intends. Those who back him are awaiting the biggest shake-up in Washington, DC, in half a century - though their optimism is an act of faith. Those who oppose him are convinced there will be chaos and ruin on an epoch-changing scale - though their despair is guesswork. All that just about everyone can agree on is that Mr Trump promises to be an entirely new sort of American president. The question is, what sort?

You may be tempted to conclude that it is simply too soon to tell. But there is enough information - from the campaign, the months since his victory and his life as a property developer and entertainer - to take a view of what kind of person Mr Trump is and how he means to fill the office first occupied by George Washington. There is also evidence from the team he has picked, which includes a mix of wealthy businessmen, generals and Republican activists.

For sure, Mr Trump is changeable. He will tell the New York Times that climate change is man-made in one breath and promise coal country that he will reopen its mines in the next. But that does not mean, as some suggest, that you must always shut out what the president says and wait to see what he does. 

Words count

When a president speaks, no easy distinction is to be made between word and deed. When Mr Trump says that NATO is obsolete, as he did to two European journalists last week, he makes its obsolescence more likely, even if he takes no action. Moreover, Mr Trump has long held certain beliefs and attitudes that sketch out the lines of a possible presidency. They suggest that the almost boundless Trumpian optimism on display among American businesspeople deserves to be tempered by fears about trade protection and geopolitics, as well as questions about how Mr Trump will run his administration.

Start with the optimism. Since November's election the S&P500; index is up by 6%, to reach record highs. Surveys show that business confidence has soared. Both reflect hopes that Mr Trump will cut corporate taxes, leading companies to bring foreign profits back home. A boom in domestic spending should follow which, combined with investment in infrastructure and a programme of deregulation, will lift the economy and boost wages.

Done well, tax reform would confer lasting benefits, as would a thoughtful and carefully designed programme of infrastructure investment and deregulation. But if such programmes are poorly executed, there is the risk of a sugar-rush as capital chases opportunities that do little to enhance the productive potential of the economy.

That is not the only danger. If prices start to rise faster, pressure will mount on the Federal Reserve to increase interest rates. The dollar will soar and countries that have amassed large dollar debts, many of them emerging markets, may well buckle. One way or another, any resulting instability will blow back into America.

Instability breeds instability

If the Trump administration reacts to widening trade deficits with extra tariffs and non-tariff barriers, then the instability will only be exacerbated. Should Mr Trump right from the start set out to engage foreign exporters from countries such as China, Germany and Mexico in a conflict over trade, he would do grave harm to the global regime that America itself created after the second world war.

Just as Mr Trump underestimates the fragility of the global economic system, so too does he misread geopolitics. Even before taking office, Mr Trump has hacked away at the decades-old, largely bipartisan cloth of American foreign policy. He has casually disparaged the value of the European Union, which his predecessors always nurtured as a source of stability.

He has compared Angela Merkel, Germany's chancellor and the closest of allies, unfavourably to Vladimir Putin, Russia's president and an old foe. He has savaged Mexico, whose prosperity and goodwill matter greatly to America's southern states. And, most recklessly, he has begun to pull apart America's carefully stitched dealings with the rising superpower, China - imperilling the most important bilateral relationship of all.

The idea running through Mr Trump's diplomacy is that relations between states follow the art of the deal. Mr Trump acts as if he can get what he wants from sovereign states by picking fights that he is then willing to settle - at a price, naturally.

Nations not corporations

His mistake is to think that countries are like businesses. In fact, America cannot walk away from China in search of another superpower to deal with over the South China Sea. Doubts that have been sown cannot be uprooted, as if the game had all along been a harmless exercise in price discovery. Alliances that take decades to build can be weakened in months.

Dealings between sovereign states tend towards anarchy - because, ultimately, there is no global government to impose order and no means of coercion but war. For as long as Mr Trump is unravelling the order that America created, and from which it gains so much, he is getting his country a terrible deal.

So troubling is this prospect that it raises one further question. How will Mr Trump's White House work? On the one hand you have party stalwarts, including the vice-president, Mike Pence; the chief of staff, Reince Priebus; and congressional Republicans, led by Paul Ryan and Mitch McConnell.

On the other are the agitators - particularly Steve Bannon, Peter Navarro and Michael Flynn. The titanic struggle between normal politics and insurgency, mediated by Mr Trump's daughter, Ivanka, and son-in-law, Jared Kushner, will determine just how revolutionary this presidency is.

As Mr Trump assumes power, the world is on edge. From the Oval Office, presidents can do a modest amount of good. Sadly, they can also do immense harm.

The Economist

The above appears as a leader in the latest edition of The Economist