His decision to attack and try and topple Iran's regime with the backing of Israel, tips the Middle East into a massive war that could last weeks.
By the end of last week, the stage had been set for more diplomacy. Talks in Geneva had been making some progress and more were planned for Monday.
That appears to have been a ruse, to lull the enemy into thinking an attack was not imminent.
Mr Trump has unleashed American military force without either the approval of the UN or the US Congress.
There has been no pretence at securing even a fig leaf of justification under international law.
He had been urged by allies in the region, apart from Israel, to avoid war.
If it leads to the end of the ayatollahs' regime, there will be jubilation among them, but that is a very big if.
Iran had warned it would take the gloves off if attacked and will most likely do just that, its leadership seeing this as a fight for survival.
There are already reports of attacks on countries and installations across the Persian Gulf.
It is a seaway through which a fifth of the world's oil passes, so a sustained conflict is likely to have major repercussions on the global economy.
The American-Israeli offensive poses a dilemma for Britain.
Prime Minister Sir Keir Starmer, worried about the implications under international law, has ruled out British bases being used to support the assault on Iran.
There will be severe misgivings at the highest level of government here about the wisdom and legality of this unprecedented move by Britain's closest ally but there will also be a determination that it does not damage transatlantic relations.
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And there will be the suspicion that this has been motivated more by Donald Trump's deepening domestic woes than by sound military thinking.
He faces an affordability crisis back home and continuing pressure over the Epstein scandal.
This is a massive gamble by an American president whose popularity is plummeting.
He has set the objective high, hoping to sufficiently damage Iran's regime so that its people can rise up and take over.
Anything short of that may be seen as an expensive failure, which could prove very costly for Mr Trump in the run-up to this year's vital US mid-term elections.
There were threats and grave warnings of retaliation and regional conflict. American military assets in the region were described as legitimate targets. Iran has now acted on those warnings.
Iran latest: follow here for updates
In scenes few imagined possible, people were filmed running for shelter in Dubai as air raid sirens sounded. At least three big blasts have been heard over the city, witnesses told Reuters.
A fire broke out near the entrance of Fairmont The Palm hotel in Dubai, after debris reportedly fell from the sky during an Iranian missile attack.
Photos on social media and videos verified by Sky News showed a large blaze and smoke rising from outside the building.
Dubai's media office said emergency response teams responded to an "incident" in a building in the Palm Jumeirah area.
It said four people had been injured and were taken to medical facilities. It also said the fire was under control, as it urged the public to remain calm.
Fairmont The Palm is described as a luxury hotel with gourmet dining options, an award-winning spa, and pools with city views.
A series of explosions were also heard in Abu Dhabi, with the defence ministry confirming one person was killed by falling debris in a residential area after the UAE "successfully" intercepted Iranian missiles.
In a statement, the nation's defence ministry "affirmed its full readiness and preparedness to deal with any threats, stressing that all necessary measures are being taken to firmly confront any attempts to undermine the country's security and stability".
"The kids are terrified", one Abu Dhabi resident messaged me after the sound of explosions. This was followed by a second wave of missiles seen flying over the residential area of Dubai, where I live.
The UAE's airspace is currently closed as a precautionary measure, according to the General Civil Aviation Authority.
Multiple airlines have cancelled flights serving Abu Dhabi and Dubai; the latter is the world's busiest travel hub, and handled nearly 100 million passengers last year.
If you look at Flight Radar, the airspace is empty in neighbouring countries too. Qatar, Jordan, Kuwait and Bahrain have all reported deploying air defences to shoot down incoming Iranian missiles.
Flights have been turned around mid-air, and packed airliners bound for Qatar were turned around on the tarmac at Heathrow and returned to the terminal.
The British Embassy in the UAE posted travel advice to British nationals: "We are aware of missiles flying over the United Arab Emirates... British nationals are advised to shelter in place."
The advice is to remain indoors or in a safe place and to stay away from windows.
Around 240,000 British citizens live in the UAE. The sense of safety, one of the great draws for people living here, has been shattered today.
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The country is also home to an estimated half a million Iranian citizens, many of whom are full of fear and emotion today.
We have no idea what this moment means for Iran or for the region. But people here are desperately hoping America knows what it's doing.
It comes as many flights in and out of the Middle East have been cancelled due to US missile attacks on Iran and subsequent counter strikes targeting American bases in the region.
A post on X by Abu Dhabi Airports said the incident "resulted in one fatality involving an Asian national and 7 injuries" - but did not give a cause and urged against "circulating rumours".
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The Dubai incident left four staff with injuries and happened when a concourse sustained "minor damage", according to the city's media office.
Aviation sources told Reuters news agency one of the terminals had been damaged during an overnight Iranian attack.
Flights from both airports were grounded on Saturday as a precaution.
There was also a minor fire on the facade of the city's iconic sail-shaped Burj al Arab hotel due to a drone interception.
"Civil Defence teams responded immediately and brought the incident under control. No injuries have been reported," Dubai's media office added on X.
Another luxury hotel in the city was also damaged by fire earlier on Saturday - again believed to have been caused by remnants from a falling missile.
The UK Foreign Office (FCDO) has warned Britons in several Middle East countries to "immediately shelter in place" and to avoid travel to Israel and Palestine.
"Remain indoors in a secure location, avoid all travel and follow instructions from the local authorities," advised the FCDO.
It said it was "working around the clock" to support British nationals affected.
UK citizens are being urged to sign up to Register Your Presence and travel advice emails so the Foreign Office can send them information.
Thousands of Britons are potentially stuck due to the temporary closure of airspace and airports in countries such as the UAE and Qatar.
British Airways cancelled Saturday's Dubai, Doha, Amman and Abu Dhabi services, as well as flights to Tel Aviv and Bahrain up to 3 March.
A BA flight from London to Doha, Qatar's capital, was among dozens forced to turn around or divert mid-flight early on Saturday, according to flight tracking websites.
London Heathrow said "a small number of flights" had been cancelled or delayed.
Its departures board on Saturday afternoon showed cancellations including an Emirates flight to Dubai, a Qatar Airways flight to Doha, and an Etihad service to Abu Dhabi.
A Gatwick spokesperson said it was also "expecting disruption to our Qatar and Emirates flights".
Manchester Airport departures showed a Saturday night Emirates flight to Dubai had been scrapped as it shared a post advising people to check with their airline.
Wizz Air also confirmed flights between London Luton to Tel Aviv were suspended in both directions until 7 March but that "operational decisions will continue to be reviewed".
Virgin Atlantic said journeys from destinations including India and the Maldives could take longer as the routes were being altered for safety reasons.
Many other large airlines have also paused flights to the Middle East, as well as some routes flying over the region, including Delta, Lufthansa, Cathay Pacific and Turkish Airlines.
India's airlines are also suffering serious disruption.
Its aviation ministry said more than 400 flights by domestic carriers had been cancelled on Saturday and a similar number were expected on Sunday.
Dubai airports at standstill
The worst disruption is likely to be caused by the grounding of flights at Dubai's main airport, the world's busiest for international traffic.
People jetting out for a winter break - and the thousands who change aircraft there - are set to have their plans upended after the UAE closed its airspace.
All flights at Dubai's International, Dubai World Central (Al Maktoum) and Abu Dhabi's Zayed International have been suspended.
At least 90,000 people transfer every day in Dubai, Doha or Abu Dhabi on just three airlines, Emirates, Qatar Airways and Etihad Airways, according to analytics firm Cirium.
People in Dubai were earlier filmed running for shelter as air raid sirens sounded and witnesses told Reuters at least three big blasts were heard over the city.
A fire broke out near the entrance of the five-star Fairmont The Palm hotel after debris reportedly fell from an Iranian missile.
Read more:
Big blasts heard over Dubai as people run for cover
Where has been hit by strikes and counter-strikes?
A Dubai airports statement called the closures an "exceptional precautionary measure", while Emirates said safety was its "highest priority" as it halted flights.
"You have crews, planes and passengers stranded all over the world. It's a massive logistical nightmare," a Gulf airline source told Reuters.
Qatar's international airport also suspended all services after the country's airspace was locked down in case of more attacks from Iran.
People were earlier forced to run for cover as a missile - which was likely intercepted - tumbled out the sky and exploded in a residential area of Doha.
America's largest regional air base, al Udeid, is near the city and Iran has deemed all US bases in the region as fair game.
The Iranian cities hit include Tehran, Isfahan, Tabriz, Kermanshah, and Qom, according to Telegram posts from the country's Revolutionary Guard Corps.
Also targeted was the residence of Supreme Leader Ayatollah Ali Khamenei, as well as the nation's intelligence ministry, its defence ministry, and its Atomic Energy Organization, Sky News understands.
Satellite imagery of the leader's compound, taken today, shows black smoke and extensive damage, with at least four buildings struck. An Iranian official said Khamenei was not there at the time.
According to the Iranian Health Ministry, more than 60 students were killed and 80 more injured in another strike at a girls' school in Minab, a city in Hormozgan province.
Iran latest: follow here for updates
Now Iran has hit back, simultaneously striking four US bases across the Middle East.
These were Al Udeid Air Base in Qatar, Al Salem base in Kuwait, Al Dhafra Air Base in the United Arab Emirates, and the US Fifth Fleet base in Bahrain.
The attack on the US Navy's Fifth Fleet centre was also reported by Bahraini state news agency BNA.
Sky correspondent Sally Lockwood further reports that a loud explosion had been heard in Dubai. Footage from different locations across the city showed where missiles had been intercepted in the skies overhead.
More video from social media showed a fire near the entrance of Dubai's Fairmont The Palm hotel, reportedly after debris fell from the sky during an Iranian missile attack.
At least one person has been killed in the UAE. Another four were killed in Syria, when a missile struck a building in the south of the country, the state-owned SANA news agency reported.
While in Kuwait, several employees at the nation's only commercial airport were injured by a drone, which also damaged Terminal 1, according to the country's Directorate General of Civil Aviation.
There have been no reported American casualties, two officials told our US partner NBC News.
The Revolutionary Guard said it considers "all US bases, resources, and interests in the region to be legitimate targets" for retaliation.
Sky News' Europe correspondent Alistair Bunkall has warned that the Tehran regime "will not roll over and will die fighting if needs be".
It comes after Donald Trump confirmed "major combat operations" against Iran in an eight-minute video on Truth Social.
The US president also urged the Iranian people to rise up against the ruling regime, saying: "When we are finished, take over your government, it will be yours to take".
He's under pressure from those on the left of British politics to condemn US President Donald Trump's actions as "illegal" and unjustified - while those on the right want to see him offer his open support.
Military intervention in the Middle East is clearly a sensitive issue for a Labour party haunted by the Iraq war.
While British planes are now in the sky to defend UK interests in the region, Starmer made it explicitly clear that such defensive activity is "in line with international law" - and that the UK wasn't involved in the American/Israeli attacks.
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Indeed, the PM is thought to have previously blocked the US from using UK bases - reportedly over concerns about breaching international law - despite Mr Trump making it very clear that he wanted to do so.
In his televised statement, the prime minister was vehement in his condemnation of the "utterly abhorrent" Iranian regime, which has "murdered thousands of [its] own people, brutally crushed dissent and sought to destabilize the region", as well as posing "a direct threat" in the UK.
While he didn't express any direct backing for Donald Trump's decision to strike Iran, he made it plain that he agreed with the rationale.
Keir Starmer concluded by urging the Iranians to "give up their weapons programmes and cease the appalling violence and repression", although he stopped short of echoing the President's desire for regime change.
The overall impression was of a UK government applauding the ends, but not necessarily the means.
Not for the first time, the prime minister is balancing on a diplomatic tightrope in order to maintain a close relationship with the White House.
He's always put the so-called special relationship at the very heart of his foreign policy - and doesn't want to risk antagonising Donald Trump (any more than he already has done recently, in refusing the use of British military bases and signing the Chagos deal with Mauritius).
Now, alongside Turkey and Kuwait, the PM has been rewarded with a Trump phone call.
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The details are sketchy; we're told they "discussed the situation in the Middle East" and the "defensive operations" being carried out by UK forces in the region".
It sounds brief, with no indication of any particular cordiality, but Downing Street will be relieved the call has happened.
The Conservatives are accusing Starmer of sitting on the fence.
But as a former human rights lawyer, Starmer has a live understanding of the legal and moral questions raised by these unilateral attacks.
As a Labour leader, he's all too conscious of the unforeseen consequences of military intervention across the world - and the chaotic uncertainty engendered by a violent transfer of power.
For now, he's decided the fence is the safest option.