Britain and other Western powers should be prepared to commit ground troops to fight against extremists such as the Islamic State group, former prime minister Tony Blair has said.
According to Mr Blair, air strikes alone will not be enough to defeat IS or similar groups.
He said that while training and equipping local fighters may work, the option of sending in combat soldiers should not be ruled out.
Mr Blair, whose premiership came to be defined by the Iraq War, acknowledged there was "no appetite" for ground engagement against IS.
But he warned: "You cannot uproot this extremism unless you go to where it originates from and fight it."
He said the struggle against Islamist extremism should be seen as an international fight rather than a series of isolated conflicts, comparing it to the fascist and communist ideologies of the last century.
In an essay on his website, Tony Blair Faith Foundation, Mr Blair also stressed the importance of engaging with a wider spectrum of radical Islamism, not just the violent fringe.
He said because extremists such as IS are "fanatical" and "prepared both to kill and to die", there could be no solution that does not involve force "with a willingness to take casualties in carrying the fight through to the end".
"This is where we get to the rub. We have to fight groups like Isis," he said.
"There can be an abundance of diplomacy, all necessary relief of humanitarian suffering, every conceivable statement of condemnation which we can muster, but unless they're accompanied by physical combat, we will mitigate the problem but not overcome it."
Air strikes launched
The US and France have already launched air strikes against IS targets, and the UK has not ruled out joining the bombing campaign against the extremists, who have occupied a large area of Iraq and Syria.
The British government has supplied arms, including heavy machine guns, to Kurdish fighters on the frontline and has also been involved in transporting materiel supplied by other countries.
But Mr Blair said: "Air power is a major component of this, to be sure, especially with the new weapons available to us. But - and this is the hard truth - air power alone will not suffice.
"They can be hemmed in, harried and to a degree contained by air power. But they can't be defeated by it.
"If possible, others closer to the field of battle, with a more immediate interest, can be given the weapons and the training to carry the fight; and in some, perhaps many cases, that will work. It may work in the case of Isis.
"There is real evidence that now countries in the Middle East are prepared to shoulder responsibility and I accept fully there is no appetite for ground engagement in the West.
"But we should not rule it out in the future if it is absolutely necessary.
"Provided that there is the consent of the population directly threatened and with the broadest achievable alliance ... we have, on occasions, to play our part."
He said the lessons learned from the wars in Afghanistan and Iraq had improved Western forces' "capacity and capability" to respond to the threat of IS and similar groups.
"To those who say that after the campaigns in Afghanistan and Iraq we have no stomach for such a commitment, I would reply the difficulties we encountered there are in part intrinsic to the nature of the battle being waged," he said.
"And our capacity and capability to wage the battle effectively are second to none in part because of our experience there.
"However, we are not talking here about armies of occupation.
"We are, in certain situations where it is necessary and subject to all proper limitations, talking about committing ground forces, especially those with special capabilities."