US President Donald Trump declared it was “mission accomplished” after the overnight bombing campaign in Syria.
And he thanked the UK and France “for their wisdom and the power of their fine military”.
Britain launched cruise missiles as part of the co-ordinated military operation with the United States and France in response to the chemical weapons attack in Douma one week ago.
Mr Trump said the joint strikes against the Syrian regime were “perfectly executed”.
He added: “Could not have had a better result. Mission Accomplished.”
Theresa May insisted that joining the military campaign was the “right thing for us to do” in the wake of the “harrowing” assault on the Syrian rebel-held town.
The Prime Minister also warned Russia that the air strikes should act as a warning to Russia over its use of chemical weapons.
Speaking at Number 10, Mrs May said action was “legal” and defended the decision to go ahead without securing the backing of Parliament.
But she was accused of “riding the coat-tails of an erratic US president” by the Liberal Democrats, and “taking instructions” from Washington by Labour.
Four Royal Air Force Tornado GR4s joined the co-ordinated missile strikes at 2am, launching Storm Shadow missiles at a base 15 miles west of Homs.
The Prime Minister said the Cabinet had taken advice from the Attorney General, National Security Adviser and military chiefs when it met on Thursday.
She added: “We agreed that is was both right and legal to take military action together with our closest allies.”
Number 10 is expected to put out a summary of the advice it received later on Saturday, Downing Street sources said.
Mrs May said “it was right we acted the way that we did” for operational security reasons to help protect the military.
She insisted the action against Bashar Assad’s regime was a limited and targeted strike to degrade and deter the Syrian government and was not about regime change.
But she also drew a link with the nerve agent attack on Russian former spy Sergei Skripal and his daughter Yulia in Salisbury.
Looking drawn as she spoke to reporters in Downing Street in a hastily arranged press conference, she said: “We cannot allow the use of chemical weapons to become normalised – either within Syria, on the streets of the UK or elsewhere.”
An emergency session of the UN Security Council, called by Russia, will meet at 4pm UK time.
Russian President Vladimir Putin said the strikes were an “act of aggression” that would have a “destructive” influence on international relations.
Asked if the action had also been a warning to Russia, Mrs May said: “The action that took place last night was an action which was focused on degrading and deterring the operational capability and the willingness of the Syrian regime to continue to use chemical weapons.
“There have been many instances when we have seen them using those chemical weapons.
“But I believe it should also be a message to others that the international community is not going to stand by and allow chemical weapons to be used with impunity.”
Jeremy Corbyn said the military action against Syria was “legally questionable” and makes real accountability for war crimes less likely.
He said: “Bombs won’t save lives or bring about peace. This legally questionable action risks escalating further, as US Defence Secretary James Mattis has admitted, an already devastating conflict and therefore makes real accountability for war crimes and use of chemical weapons less, not more likely.”
Mrs May has faced criticism from across the political spectrum for failing to recall Parliament and put the plans to a vote.
The Prime Minister said she will go before the Commons on Monday to answer questions about her decision but insisted there was no “alternative path”.
On Friday, Russian military chiefs claimed they had evidence the UK had directed the attack in Douma using the White Helmets, a group of humanitarian volunteers on the ground.
Mrs May said the accusation was “grotesque and absurd” as she criticised Russia for vetoing at the UN calls for an independent investigation.
The PM did not rule out further action if Syria continues to use chemical weapons.
Mr Trump said the Syrian regime’s use of chemical weapons on Douma was a “significant escalation in a pattern of chemical weapons use by that very terrible regime”.
Following the announcement, the US said strikes had been launched at 9pm EST (2am BST) and had destroyed important infrastructure at three sites connected with the Syrian regime’s chemical weapons programme.
General Joseph Dunford, chairman of the US Joint Chiefs of Staff, said the first was at a scientific research centre in greater Damascus, involved in the development and production of chemical warfare.
Other strikes targeted an army depot near Homs.