This is a man who feels at home with violence. As a young paratrooper, he hugged his commanding officer who was engulfed in a tower of flame until the fire was extinguished. As one of the finest generals of his generation, he used industrial strength force against Saddam Hussein and Slobodan Milosevic. And with his bomber jacket and masculine good looks — Jeremy Irons with a shot of testosterone — General Sir Rupert Smith could be a Hollywood action hero rather than an old British war dog.

This human fighting machine, plastered with more decorations for gallantry than Tony Blair has been with rotten eggs, is about to spring his own offensive in the literary world. “War,” he declares “no longer exists.” Violence exists; indeed, it flourishes. But war will not be waged and won between states. Instead we will fight “among the people”. The enemy will be tottering regimes and terrorism. To defeat them, we might even “franchise” the fighting to local warlords.

The problem, Smith suggests, is that governments and their armed forces haven’t really noticed the shift yet. So they maintain vast monolithic military structures as absurd and anachronistic in their way as the Maginot line.

Our leaders no longer employ violence efficiently, says Smith, so every conflict continues in confusion without conclusion. He warns that because of the disastrous lack of planning, Iraq will not end in anything that can be trumpeted as “victory”, prompting soldiers to quit Britain’s armed forces more swiftly than Australia’s finest departed the crease this summer. Until ministers swap their Armani suits for flak jackets and join the troops on the front line, Smith suggests, their political objectives will never be properly formulated, let alone achieved.

Having retired in 2002 he sets out his controversial theory in a book, The Utility of Force: The Art of War in the Modern World, that is sure to let off a few explosions of its own. He rather enjoys the flak, but in these dangerous days he must plan raids carefully. We meet at his house in a foreign capital that he asks us not to reveal.

“I have finally got rid of the men in black with earpieces and I don’t want them back,” he says, peering intensely with boyish blue eyes. He clearly relishes the freedom that goes with exchanging an army commission for a book commission. He was never a soldier who instantly clicked his heels to authority: he was too clever. His school report read: “A fair amount of discipline still required.” Even as a general he could be subversive. When ordered to write a CV, he made it read like his career had been a fiasco.

Rebelling against years of convention, his life now seems rather bohemian. After a long marriage that produced two children, he lives with Ilana, a raven-haired Israeli-born intellectual whom he met while blasting Slobo out of Bosnia, where he commanded United Nations forces.

The couple have a five-year-old son and there is still a little of the Famous Five in Smith, 61, as he darts between trams in his tiny car, but this is a cerebral soldier. General Sir Mike Jackson’s catchphrase is “Let’s crack on”; Smith is more likely to say, “Let’s crack heads together”. While soldiers revel in their latest gizmos, Smith says the most effective weapon of recent years has not been the smart bomb but the dumb old machete. Take Rwanda, where it killed tens of thousands. How else, pray, should you measure a weapon’s success?

Smith’s manner may be gentle but he really does have a masterful understanding of violence. It is probably in the blood: his father, a New Zealander, was a battle of Britain pilot who blew up the Gestapo HQ at Amiens. His great-uncle was one of the few survivors of Captain Scott’s ill-fated expedition to Antarctica.

“From Robert Capa’s photographs to television news, we have equated technology with the use of force,” he says. “Planes and tanks are iconic images. But force can be achieved by throwing rocks.”

Soldiers feel obliged to use all their toys, but does this advance their objectives? In Iraq, says Smith, they patrol in tanks, trying to establish normality but actually signalling to locals that the situation is anything but normal — and that the people are their enemies. If you treat folk like enemies, they become enemies.

“We don’t understand how to use the weapons we have,” sighs the ex-para, who won plaudits from American generals as commander of the 1st Armoured Division chasing Saddam’s forces back towards Baghdad in the first Gulf war.

Despite billions spent on defence, we are almost back where we started: defenceless. When a terrorist can detonate a bomb with a mobile phone, our battalions are useless: “Terrorists don’t have their own transport system. They use yours and adapt some of it into a weapon to attack you.”