I have been up all night at the American Embassy in London witnessing the result of what looks like the most important election of my lifetime and maybe since the Second World War.

Because there is something profoundly shocking that the world's richest, most powerful nation - a proud old proselytizing democracy - is on the verge of choosing Donald Trump as its new president.

There are so many dimensions to this decision by American citizens that it is hard to know where to start.

But let's begin by noting that never in history have voters in the US or any developed western country chosen a candidate to the their leader who boasted on camera about violating women, was accused by ten women of abusing them, swaggered about minimising his tax payments and exploiting bankruptcy laws, threatened to put his opponent in jail, warned he might not respect the result of the election, and promised policies that to many look racist.

Millions and millions of Americans either didn't believe his character is as flawed as it seems, or didn't care. So if we needed evidence that vast numbers of them, especially white men, feel utterly alienated from the political establishment, well we have it now.

That palpable alienation of a country from its venerable and supposedly revered institutions - or at least the establishment representatives of those institutions - is quite simply momentous.

It shows, as that Brexit vote showed in this country, that there is a very angry mob who - rightly - believe the economic and political system serves only to enrich others.

So what kind of president will he be? Well during the campaign he flip-flopped on many of his pledges, so we can't be sure.

But he said he will put up barriers to free trade, especially exports from China and Mexico - which is why the Mexican Peso is collapsing as I write.

He has said he will be tougher on immigration, especially of Muslims, and has some unspecified secret plan to defeat so-called Islamic state.

He has expressed admiration for Putin, while implying that his foreign policy will be more isolationist than most of his predecessors. For what it's worth, Trump's admirer Nigel Farage tells me Trump hates the European Union even more than he does.

What does all this mean for us?

Well it's too early to say. But it won't be trivial.

One stress-inducing question is whether Putin will test Trump's apparently lukewarm commitment to deploying US troops by ordering his own armed forces into the Ukraine and nearer the Baltic states - to consolidate Russian regional power?

Another is whether China under President Xi be emboldened too to consolidate its military presence in east Asia, and increase already serious tensions with Japan.

Much more immediately, there'll be something of a global financial shock. Share prices are set to tumble today. The dollar will weaken.

Little wonder that the British ministers and officials I've been with at the embassy were dazed and confused.

The point is that it's almost impossible to find an MP from any party who wanted Trump to win.

So our prime minister and her advisers will today have to use all their creative skills to craft a statement that will highlight the importance of sustaining the so-called Special Relationship between Britain and America, while glossing over her government's conspicuous reservations about him.

In a nutshell, the world seems a lot scarier and less predictable this morning than it seemed last night.

And at the risk of infuriating passionate Brexiteers, I wonder whether the merits of integration with the rest of the European Union - as a bulwark against the uncertainties to our east and now possibly to our west - will feel quite as toxic to many Britons as was the case just a few weeks ago.

The arrival of the Donald is set to challenge quite a lot of what we think we know about ourselves.