In 2013, Parliament defeated a call by then-Prime Minister David Cameron for air strikes in response to an earlier chemical attack in Syria.
Which is why the Syrian crisis, and the whole debate over Britain's participation in military action against the Assad regime, has forced Mrs May out of her political comfort zone and compelled her, for once, to provide some decisive leadership.
Sky News reported that May was expected to ask her ministers on Thursday to approve Britain's involvement in military action against Syria's chemical weapons infrastructure.
According to the White House, US President Donald Trump has not laid out any timetable for action, either. "Could be very soon or not so soon at all!" he tweeted, without ruling out attacks. They should have a voice in this.
"It is worth noting, however, that even if military action is taken before MPs return from their Easter recess, May will nearly certainly have to explain her response to parliament and seek its approval for any longer-term engagement", he adds. SNP leader at Westminster Ian Blackford said: "There is no mandate for the Government to take this action".
Britain has been working with its allies to determine what happened, and "we are rapidly reaching that understanding, " she said during a visit to the English city of Birmingham.
Adding: "But also, it's a very, very delicate circumstance and we've got to make this judgment on a very careful, very deliberate, very well thought-through basis".
"The results of the analysis by the OPCW designated laboratories of environmental and biomedical samples collected by the OPCW team confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury and severely injured three people", the independent body said.
Arm wrestling goes on, but Trump demurs on Syrian retaliation
Assad warned on Thursday that threats of Western military action "will only contribute to further destabilisation in the region". The Syrian military has also been repositioning some air assets to avoid possible missile strikes, U.S. officials told Reuters.
The Ministry of Defence refused to comment on a report in the Daily Telegraph that Royal Navy submarines had been ordered into range to launch Tomahawk cruise missile strikes as early as Thursday night.
Mr Trump's latest intervention came after he previously tweeted that missiles "will be coming".
May will hold an unscheduled meeting of her cabinet on Thursday, which will involve senior ministers, her spokesman told media.
Moscow has advised the U.S. against attacking Syria and is calling for an investigation by the OPCW to determine if chemical weapons were indeed used in Douma. We can not risk an escalation even further than it's gone'.
He wrote on Twitter, 'Russia vows to shoot down any and all missiles fired at Syria. "You shouldn't be partners with a Gas Killing Animal who kills his people and enjoys it!"
Foreign secretary Boris Johnson said: "There can be no doubt what was used and there remains no alternative explanation about who was responsible - only Russian Federation has the means, motive and record". The Syrian government has denied the allegations.
Government sources allegedly told the publication that Britain is "doing everything necessary" to put the submarines in a position to fire missiles at military targets in Syria.
It came nearly exactly a year after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people.