Air Commodore Johnny Stringer claimed IS morale is “crumbling” following the liberation of Mosul in Iraq and Raqqa, its Syrian HQ.
His optimism was echoed by the new Defence Secretary, Gavin Williamson, who said the campaign against IS was in its last stages.
Amid hand-wringing in Britain about how to treat returning jihadis, Air Cdre Stringer said the idea they had not known what they were joining was “rubbish.”
Having just returned from a year commanding Britain’s air contribution to the US-led Coalition, he told a London briefing that jihadis were legitimate targets after flocking to “nihilistic” IS.
About 850 Britons are known to have gone out to IS – or Daesh.
Some 150 have been killed; 400 have come home, leaving 300 still fighting on.
Last month Foreign Office minister Rory Stewart said the only way to deal with jihadis is to kill almost every one.
But the independent reviewer of terrorism legislation, Max Hill QC, said young people who travelled to Syria after being “brainwashed” should be allowed to come home and rejoin society.
The Government is now considering a pilot project which would give returning jihadis council houses and other support to try to de-radicalise them.
But Air Cdre Stringer said the jihadis had made their choice.
He added: “I think the key part is that you have got British passport holders, British citizens, who by their free will have elected to go out and frankly fight on Daesh’s behalf. Let’s not sort of dress this up. They have made themselves valid targets in so doing.
“I think any sense that you could go out and have some sort of Daesh-lite activity is rubbish frankly and equally any sense that you could go there and not know at heart just what a dark organisation it was – and what it was conducting in Iraq and Syria and its longer term intent – is probably fanciful.
“You pitch up in Iraq or Syria fighting for Daesh, then you are absolutely valid to be targeted by the Coalition.”
The rule of IS has been brutal, with public executions often used for propaganda.
Air Cdre Stringer added: “I cannot think of anybody so nihilistic as Daesh. For the hard core of their members that we have seen in Iraq and Syria, there has been no discussion, because they want to fight to the death and take as many people with them as they can.”
However he added: “The caliphate has all but disappeared. Around 90 per cent of territory captured by Daesh has been liberated and 5.6 million people previously under their yoke freed.
“Daesh morale has been crumbling for many months. We are clearly in the end game now. That is not to say it is over – and that is not to say there is not going to be some hard fighting to come.”
New Defence Secretary Gavin Williamson agreed, saying: “As the campaign… enters its final phase, we will continue to support the international coalition to decisively defeat our common enemy in Iraq and Syria. It is only by defeating them and their poisoned ideology that we can keep Britain safe.”
Air Cdre Stringer also said the Coalition has thwarted UK terror attacks by blasting IS positions in the Middle East or disrupting their communications.
But he said the UK is still at risk.