A series of missile strikes have been launched against Syria by the US, UK and France in response to the suspected chemical weapons attack in Douma a week ago.
The strikes began at 2am BST and destroyed important infrastructure at three sites connected to the Syrian regime's chemical weapons programme, according to the allies.
US Defence Secretary James Mattis said double the number of weapons were used in the strikes compared with the 2017 Shayrat missile strike which involved 59 US Tomahawk cruise missiles and was launched in response to the Khan Shaykhun chemical attack.
There were no reports of any allied losses during the latest strikes and only limited resistance in the form of some Syrian surface-to-air missile activity.
Syria's main backer Russia, which has insisted there was no chemical weapons attack in Douma and accused the West of fabricating evidence of it, was not informed of the strikes in advance.
Announcing the operation, Prime Minister Theresa May said there was "no practicable alternative to the use of force".
Mrs May said "every possible diplomatic channel" had been explored before authorising the strikes, adding that it was not a decision she had taken lightly.
She said: "This persistent pattern of behaviour must be stopped - not just to protect innocent people in Syria from the horrific deaths and casualties caused by chemical weapons but also because we cannot allow the erosion of the international norm that prevents the use of these weapons.
"This is not about intervening in a civil war. It is not about regime change.
"It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties."
US President Donald Trump said the Syrian regime's use of chemical weapons on Douma last Saturday was a "significant escalation in a pattern of chemical weapons use by that very terrible regime".
Giving a statement at the White House, Mr Trump said: "This evil and despicable act left mothers and fathers and children thrashing in pain and gasping for air.
"The combined American, British and French response will integrate all instruments of our national power."
Mrs May said the action would also send a "clear signal" to anyone else who believed they could use chemical weapons "with impunity".
She said: "This is the first time as Prime Minister that I have had to take the decision to commit our armed forces in combat - and it is not a decision I have taken lightly."
Mrs May authorised the strikes despite demands from opposition parties that Parliament was consulted before any military action was launched.
But the Prime Minister said she had authorised the operation "because I judge this action to be in Britain's national interest".
"We cannot allow the use of chemical weapons to become normalised - within Syria, on the streets of the UK, or anywhere else in our world," she added.
French president Emmanuel Macron said there was no doubt the Syrian regime was responsible for the chemical attack in Douma.
He said: "We cannot tolerate the trivialisation of the use of chemical weapons, which represent an immediate danger for the Syrian people and for our collective security.
"The red line set by France in May 2017 has been crossed."
There was no immediate military response from Moscow but Russian politicians reacted angrily, with one MP likening Mr Trump to Adolf Hitler.
General Joseph Dunford, chairman of the US Joint Chiefs of Staff, told reporters at a Pentagon briefing that the first of the allied strikes on Saturday was at a scientific research centre in greater Damascus involved in the development and production of chemical weapons.
The second was at a chemical weapons storage facility west of Homs, while the third was at a chemical equipment storage facility and important command post.
He added: "Important infrastructure was destroyed which will result in a set-back for the Syrian regime. They will lose years of research and development, storage and equipment."
Four Royal Air Force Tornados contributed to strikes, launching Storm Shadow missiles at the target 15 miles west of Homs in a "successful attack", the Ministry of Defence said.
The MoD said the Syrian regime was "assessed to keep chemical weapon precursors stockpiled in breach of Syria's obligations under the Chemical Weapons Convention" at the site.
Defence Secretary Gavin Williamson said: "The reprehensible use of chemical weapons in Douma is further evidence of the Syrian regime's appalling cruelty against its own people.
"We will not stand by whilst innocent civilians, including women and children, are killed and made to suffer.
"The international community has responded decisively with legal and proportionate military force.
"Let these united actions send a clear message to the regime - the use of chemical weapons is categorically unacceptable and you will be held to account."
General Dunford said the strike had inflicted maximum damage without unnecessary risk to civilians.
Asked if the US and its allies would continue military operations, he said: "That will depend on Mr Assad should he decide to use more chemical weapons in the future, and of course the powers that have signed the Chemical Weapons Prohibition (Organisation for the Prohibition of Chemical Weapons) have every reason to challenge Assad should he choose to violate that.
"But right now this is a one-time shot and I believe that it sent a very strong message to dissuade him, to deter him from doing this again."
Mr Mattis said the US remained in "close consultation" with its allies, adding: "We review the evidence all the time. But right now we have no additional attacks planned."
He also praised the "level of trust" between the US, UK and France, saying: "These allies, the American, the French, the British, we have operated together through thick and thin, through good times and bad, this is a very, very well integrated team.
"Wherever we operate, we do so with complete trust in each other, but more than that the belief that one another will be there when the chips are down. It's a statement about the level of trust between our nations."