Soft words, big stick: The Prime Minister would be wise to follow the advice of Roosevelt in his response to the murder of Alan Henning

David Cameron’s insistence that Britain will do “whatever it takes” to hunt down the killers of Alan Henning is an understandably robust and human response to a crime that has outraged people, not only in Britain but across the world. Confronted by the latest evidence of the barbarism of the terrorist group Isis, it is natural for leaders to want to show that they share people’s feelings of revulsion – and have some convincing plan of action up their sleeves.

But the Prime Minister must be careful not to indulge false hopes. Talk of bringing the killers of Mr Henning to justice raises expectations that he will be hard put to satisfy. Mr Cameron’s revelation that he has met intelligence chiefs at Chequers to discuss the plight of the other hostages held by Isis has also sparked a good deal of speculation about commandos parachuting into enemy territory and staging an Entebbe-style rescue mission.

Both Britain and the US are reported to be considering ways to “take out” key figures within Isis, starting with the gloating executioner who has featured in the notorious beheading videos, the man frivolously nicknamed “Jihadi John”.

All of this is fantasy. The sooner that we disabuse ourselves of the notion that some elite unit, such as the SAS, can either resolve the terrible plight of these hostages or knock out the nerve centre of Isis through a couple of choice assassinations, the better.

Instead of repeating the same empty-sounding threats every time a hostage is killed, Mr Cameron should spell out the hard, uncomfortable truth, which is that no military action that Britain undertakes now will have any effect on the fate of Isis’s hapless foreign prisoners. The fact that these people executed a selfless humanitarian aid worker even after many internationally well-known Muslim preachers and scholars begged for his life shows that neither the personal qualities of the hostages, nor the credentials of those who plead for them, matter to them. Indeed, Mr Henning’s death shows that they still see the executions and the accompanying videos as serving an important strategic purpose – terrifying their opponents, thrilling a hard core of religious fanatics and provoking Western governments into ill-considered words and deeds.

Aside from the question of the hostages, the Prime Minister also owes it to people to be much clearer than he has been on the subject of Britain’s and the West’s ultimate goal in the conflict with Isis.

This new force in the world is not the creation of a handful of charismatic personalities, holed up in a cave in a desert, waiting to be zapped. An inchoate movement rather than a state, now controlling an area about the same size as Jordan, it will almost certainly never be defeated in the conventional military sense.

Mr Cameron should spell out the fact that all we can do is contain the threat that Isis undoubtedly poses by arming and training various proxy forces in the region, starting with the Kurds in north-west Iraq, the Iraqi army in central and southern Iraq, and the moderate opponents of the Assad regime in Syria.

With the assistance of US-led air strikes, these local armies may be able to stop Isis and even push it back a little. But that is as far as matters will go, unless the US administration embarks on a completely new strategy, which looks unlikely, or unless the Sunnis of Iraq and Syria rise up and expel the militants from within, which also looks unlikely.

No one is suggesting that Mr Cameron should sound a timid, defeatist note when it comes to Isis and its odious actions. However, it is always better to “speak softly and carry a big stick”, as President Theodore Roosevelt once counselled, than do the opposite.