Well, that didn’t take long. When I boarded a flight for New York on Thursday, Donald Trump and Elon Musk were still sticking to the script: though the dizzy days of their bromance might be over, the separation was amicable.

The most powerful man in the world and the richest even staged a chummy farewell in the Oval Office last week to underline the point. Yet by the time I’d landed at JFK just over seven hours later, Washington’s version of Krakatoa had erupted.

Right from the start of the Trump-Musk love-in I predicted it would end in tears, much to the chagrin of the President’s cult, which rounded on me with its default social media smear: ‘Trump derangement syndrome’.

But there was no way these two men with narcissistic egos the size of Saturn and their very own destructive pathologies would stay best mates for long. So it has proved. But even I didn’t foresee the speed and ferocity with which it would all unravel.

Thursday started routinely enough. Trump sat down around noon in the Oval Office with Germany’s new Chancellor, Friedrich Merz, in front of the assembled Washington Press corps to discuss trade, defence spending, Ukraine.

But within minutes, after a reporter asked the President about Musk’s attack on Trump’s signature legislation – his ‘big, beautiful bill’ to roll over existing tax cuts, add new ones and increase spending on the military and border security – hostilities broke out with a vengeance.

This was Merz’s lucky day. Instead of the German Chancellor getting the Oval Office hair-dryer treatment (as inflicted on the presidents of Ukraine and South Africa during their sit-downs), all Trump’s bile was reserved for Musk. Merz had the sense to stay schtum, relieved to be a mere spectator, albeit with a ringside seat.

‘I’ve always liked Elon,’ said Trump, unconvincingly. ‘Elon and I had a great relationship. I don’t know if we will any more,’ he added, before saying he would have won last November without Musk’s help and that the multi-billionaire was miffed because the ‘big, beautiful bill’ ended various green subsidies on which some of Musk’s businesses had thrived.

There was no way these two men with narcissistic egos the size of Saturn and their very own destructive pathologies would stay best mates for long, writes Neil

Talk about ‘light blue touch paper and retire’, as they advise on all good fireworks.

In a series of rapid machine-gun posts on X (formerly known as Twitter, which he owns), Musk threw caution to the wind, like a clown running into a minefield.

He wanted Trump impeached, claimed his tariffs would induce a recession, insisted Trump would not have been elected but for his support, threatened to dismantle a vital piece of space equipment on which the US government depends and, just for good measure, alleged that Trump’s name was on key documents related to convicted sex-trafficker Jeffrey Epstein, which is why they have not been published.

By Thursday night, as I was landing in New York, Trump was toying with cutting off government contracts to Musk’s companies and had gone to Trump Defcon One (full caps mode on his social media platform, Truth Social), saying Musk was ‘CRAZY’ and suffering from ‘Trump derangement syndrome’ (I guess I’m in good company – or not). Yesterday aides of both men tried to de-escalate the situation. I suspect there’s more chance of peace in Ukraine.

This very public spat has been devoured by folks in America and beyond as a welcome source of entertainment in grim times. I understand stocks of popcorn are already running low in some US supermarkets.

But the high-powered pyrotechnics have obscured what sparked it all off — Musk’s attack on Trump’s ‘big, beautiful bill’ as a ‘disgusting abomination… a massive, outrageous, pork-filled Congressional spending bill’ that would significantly increase the federal budget deficit and the US national debt. And on this, putting aside the hyperbole, Musk is absolutely right.

America’s debt and deficit are already bad enough. Trump will make them worse. US national debt is now over $36trillion (£27trillion), around 125 per cent of America’s massive GDP (by far the biggest in the world). The federal budget deficit is now over $2 trillion a year and rising.

There is no will on either side of the political divide in Washington – where fiscal conservatism is a dead duck – to do anything about this. Or to stop Trump from fanning the flames of a fiscal position already on fire. It is Liz Truss on steroids.

Trump holding a key in the Oval Office in May. This year, not even counting what the US president will have to borrow to pay for the tax cuts and extra spending, the US will need to refinance $7trillion of its existing debt. That’s over twice Britain’s GDP

Trump’s ‘tax-cuts-and-more-spending’ proclivities will add another $2.5-$3trillion to deficits over the next decade, taking them to over 7 per cent of GDP for as far as the eye can see. The national debt is projected to move into Italian territory – around 135 per cent of GDP or more – some time in the next decade.

Unlike the Japanese, Americans don’t save anything like enough to finance their debt and deficit from their own resources. There was a time, in the aftermath of the Second World War, when net savings accounted for 11 per cent of GDP. Today it’s a mere 0.6 per cent. So the US must borrow from the global debt markets, where it is already gobbling up a huge chunk of available lending.

This is where the rest of us – especially Britain – are adversely impacted. The more voracious the US appetite for debt, the less lending is available for the rest of the world, which pushes up the cost of borrowing for everybody else.

Britain is already paying more to borrow than any other major market economy because our fiscal position is also precarious. As America’s need for even more borrowing rises, so the cost of British borrowing will rise too.

Chancellor Rachel Reeves is awash in problems of her own making. But her plight is all the worse because of this grim global backdrop. It would be prudent to call a halt to any further tax, spending or borrowing initiatives until we can see more clearly the global impact of Trump’s reckless fiscal policy. But, of course, that sort of prudence is not in Labour’s DNA.

This year, not even counting what Trump will have to borrow to pay for the tax cuts and extra spending, the US will need to refinance $7trillion of its existing debt. That’s over twice Britain’s GDP.

This was debt incurred when interest rates were abnormally low. Now America will have to pay at least the 4.4 per cent the markets are demanding for ten-year US Treasury bonds. No wonder the cost of servicing US debt is soaring.

The annual interest on US debt is now over $1trillion. It is already the second-highest federal outlay after pensions (called social security here) and destined to rise much higher as America borrows even more at higher rates. Britain is going the same way.

Our annual interest bill for debt service is over £100billion, almost twice what we spend on defence. Last weekend, Reeves was boasting that Labour would spend £300billion more overall than the Tories planned before the end of the decade. She did not mention it meant extra borrowing on a massive scale at a time when the bond markets are skittish and the cost of borrowing for governments is rising. Like Trump, Reeves is also playing with fire.

It is scary that America is so hooked on debt despite a healthy economy. It’s usually okay to borrow big in a recession to pump-prime the economy. But not when you’ve had years of growth and full employment, which America has. It explains why all the major ratings agencies have downgraded America’s triple-A credit rating.

The biggest economic risk now to the global economy is that the US tilts into recession, which would add hugely to its already ballooning debt and deficit as tax revenues dried up. A sudden, sustained rise in real interest rates would follow, exporting a deep US recession to the rest of us.

You go bankrupt gradually, then suddenly, to paraphrase a character in one of Ernest Hemingway’s novels. The same has turned out to be true for the demise of the Trump-Musk bromance – and it is even more true for the trajectory of most fiscal crises.

We are in the gradual phase for now, allowing politicians to look the other way and tell us there’s no cause for concern. But fasten your seatbelts. That sudden deterioration could be just round the corner.