British Prime Minister Theresa May faced a backlash from the domestic opposition after launching military strikes Saturday on Syria without consulting parliament.
As the Conservative leader explained her rationale for the airstrikes, opposition parties claimed the attacks were legally dubious, risked escalating conflict and should have been approved by lawmakers.
The shadow of the 2003 invasion of Iraq still lingers in the corridors of Britain's parliament, when MPs backed then-prime minister Tony Blair in joining US military action.
"Bombs won't save lives or bring about peace," said Jeremy Corbyn, the veteran leftist leader of the main opposition Labour Party.
"This legally questionable action risks escalating further... an already devastating conflict.
"Theresa May should have sought parliamentary approval, not trailed after Donald Trump."
Stop the War, a pacifist coalition once chaired by Corbyn, has called a demonstration outside the British parliament on Monday to protest the strikes.
The group said it "strongly condemned" the action and accused May of "sanctioning killing" at US President Donald Trump's behest.
Often when the British government decides on military action, the opposition offers its full support. However, that has been less the case in recent years.
British MPs voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
It was the first time parliament had voted against a British government taking military action.
David Cameron, who was prime minister in 2013, tweeted on Saturday: "As we have seen in the past, inaction has its consequences".
Lawmakers backed action in Iraq in 2014, and again in Syria in 2015, strictly limiting strikes in both countries to targets of the Islamic State jihadist group.
The rush to swift action following an alleged chemical weapons attack on the rebel-held town of Douma on April 9 was condemned by several British opposition parties who wanted parliament recalled.
It does not sit again until Monday, when May will face MPs' questions on the strikes.
May held an emergency cabinet meeting on Thursday to discuss a potential military response.
After hearing legal, security and defence advice, she concluded it was "right and legal" to take military action.
Deploying the armed forces is a prerogative power, meaning the prime minister can launch action without backing from MPs.
But after the Conservatives entered office in 2010, the government suggested that since the 2003 vote on the war in Iraq, a convention had emerged that MPs should have a say, except in cases of emergency.
In attacks alongside US and French allies on Saturday, four British Tornado jets fired Storm Shadow missiles at a Syrian military base suspected of holding chemical weapons ingredients. The strikes at 0100 GMT were 24 kilometres west of Homs.
Scottish First Minister Nicola Sturgeon said Syria's use of chemical weapons could not be tolerated but questioned whether the strikes would halt their use or contribute to ending the civil war.
"This action risks not just further escalating the civil war in Syria but also a dangerous escalation of international tensions," said the leader of the left-wing Scottish National Party, the third-biggest force in the British parliament.
"There must be urgent confirmation from the prime minister that there will be no further action... without a full parliamentary debate."
Vince Cable, leader of the Liberal Democrats, the fourth-biggest party in parliament, accused May of "riding the coat-tails of an erratic US president".
"It shows a weak government putting short term political expediency before democracy and in so doing further diminishing the standing of Britain in the world," Cable said.
Before Saturday's attacks, Kenneth Clarke, the Conservative former finance minister, insisted that failing to consult MPs first would be "a very retrograde step" that made parliamentary accountability "pathetic".
However, there was some support for May.
The centre-right Conservatives rely on the support of the Democratic Unionist Party, Northern Ireland's biggest party, for a majority in parliament.
DUP leader Arlene Foster said she had spoken to May and the airstrikes were "limited but proportionate and justified".
Nigel Dodds, the DUP's leader in parliament, said May had the full authority required to carry out the strikes.
"We reject any suggestion that she was not entitled to do so," the party's deputy leader said.
Tom Tugendhat, the Conservative chairman of parliament's foreign affairs scrutiny committee and a former army officer, said May had "taken the correct decision".