Blair, who dispatched ground forces to Iraq and Afghanistan, acknowledged the challenge of deploying ground troops to the region once again. He wrote: “The pain of the casualties in such engagements weighs heavy. Western armed forces are mainly volunteers. We need an open and frank discussion with them about how to create the conditions to go and conduct these campaigns. There will have to be new methods of coalition building and cooperation between nations.”

The former prime minister issued a stark warning of the dangers of failing to defeat Isis. He wrote: “We will have periodic but increasingly frequent acts of terrorism that will result in many more victims and start to destabilise our political and social cohesion. Eventually the terrorists will commit an act of such size and horror that we will change our posture; but by then the battle will be much harder to win without measures that contradict our basic value system.”

He said the military fight against Isis should be part of a wider strategy to confront what he called the “perversion” of the Islamic faith by the ideology of Islamism. He sad people should be careful talking about tackling violent extremism because Islamism was a “much broader problem of ideology”, whose supporters run into the millions across the globe.