LONDON — British lawmakers voted by a wide margin Wednesday to join the international campaign of airstrikes against the Islamic State militant group in Syria, after Prime Minister David Cameron asserted that bombing the "medieval monsters" in their heartland would make Britain safer.
The 397-223 vote in the House of Commons means Royal Air Force fighter jets — already operating against ISIS in neighboring Iraq from a base in Cyprus — could be flying over Syria within days or even hours.
Anti-war protesters outside Parliament booed as they learned the result of the vote. The decision came after an emotional 10 1/2-hour debate in which Cameron said that Britain must strike the militants in their heartland and not "sit back and wait for them to attack us."
Opponents argued that Britain's entry into Syria's crowded airspace would make little difference, and said Cameron's military plan was based on wishful thinking that overlooked the messy reality of the Syrian civil war.
Cameron has long wanted to target ISIS in Syria, but had been unsure of getting majority support in the House of Commons until now. He suffered an embarrassing defeat in 2013 when lawmakers rejected a motion backing attacks on the forces of Syrian President Bashar Assad.
The mood has changed following the Nov. 13 Paris attacks, claimed by ISIS, that killed 130 people. Both France and the U.S. have urged Britain to join their air campaign in Syria, and Cameron said Britain should not let its allies down.
He said Britain was already a top target for ISIS attacks, and airstrikes would reduce the group's ability to plan more Paris-style carnage.
"Do we work with our allies to degrade and destroy this threat and do we go after these terrorists in their heartlands, from where they are plotting to kill British people?" he said. "Or do we sit back and wait for them to attack us?"
He said that attacking ISIS was not anti-Muslim but "a defense of Islam" against "women-raping, Muslim-murdering, medieval monsters."
Cameron was backed by most members of his governing Conservative Party — which holds 330 of the 650 Commons seats — as well as members of the smaller Liberal Democrat party and others.
Labour, the main opposition, was divided. Labour leader Jeremy Corbyn — who represents the left wing of the party — spoke against what he called a "reckless and half-baked intervention." But more than 60 Labour lawmakers, including senior party figures, voted in support of airstrikes, a move likely to make fissures between the right and the left of the party even worse.
Labour foreign affairs spokesman Hilary Benn said Britain could not "walk by on the other side of the road" when international allies were asking for help against ISIS "fascists."
Britain already conducts airstrikes against ISIS targets in Iraq, and in August launched a drone strike that killed two British ISIS militants in Syria.
British officials say Royal Air Force Typhoon and Tornado fighter jets, armed with Brimstone missiles capable of hitting moving targets, would bring the campaign highly accurate firepower and help minimize civilian casualties.