Last week’s murder of US journalist James Foley was shocking – as Islamic State (Isis) no doubt intended it to be. The risk is that UK policy, while right in its instinct to act against Isis, is drawn into precisely the wider confrontation that Isis desires.

That risk is real, as Foley’s killing may well encourage the public and MPs to endorse an approach to action against Isis that David Cameron proposed in an article for the Sunday Telegraph earlier this month. While the prime minister is clear that the UK’s response to Isis is not the “war on terror”, he locates a UK response within a wider conflict that resembles the war on terror.

Hard political and military goals, the achievability of which can be assessed by strategic planners (and indeed MPs) are replaced with open-ended commitments to a conflict with no clear geographical, chronological or legal boundaries that occupies a grey zone between war and peace.

Rather than clear political goals, Cameron proposes “a struggle for decency, tolerance and moderation in our modern world” – and rather than clear military goals “a battle against a poisonous ideology that is condemned by all faiths”.

We’re told in the prime minister’s article that the geographical limits of this conflict are global. This would not be a limited UK response against one particularly vicious group of Islamic extremists, but intervention in a wider conflict that incorporates as opponents all those who “pervert the Islamic faith as a way of justifying their warped and barbaric ideology … right across the world, from Boko Haram and al-Shabaab to the Taliban and al-Qaida”.

We’re told that the chronological limits of the conflict are expansive: “We are in the middle of a generational struggle against a poisonous and extremist ideology.”

We’re not told about the legal boundaries of the conflict at all – who’s the enemy and who’s not, who can be spied on and who can’t, and so on – so who knows where they start and end.

Cameron states that the danger to national security means that we “have no choice but to rise to the challenge”.

Wrong: the British public does have choices.

First, the public can choose what level of risk to tolerate. Cameron does not say that Isis is an existential threat to the UK; rather the risk is defined as its “murderous intent”, which brings to mind 7/7 – the type of attack one has to be very unlucky to be caught in.

That leaves space for an approach summarised by the US libertarian Nick Gillespie, that terrorism is “a chronic condition, but it won’t kill us. Just keep the attacks to a minimum.” If the British public choose that they want their government to aim for total security through an approach that engages the UK in a generational global conflict against all jihadists under the sun to “defeat the terrorist threat at source”, rather than accept latent risk and manage it, fine. But there plainly is a choice.

Second, if action against Isis is actually more about the stability of the Middle East, then the public has another choice: there is clearly a debate to be had about the extent to which the UK should take ownership of that problem.

Third, there is a choice about how broadly to characterise the conflict within which the UK takes action against Isis. A narrow characterisation would be limited to action exclusively against Isis, on the basis of a specific threat to the UK, and perhaps additionally, regional stability in the Middle East.

The broad characterisation that Cameron proposes frames the fight against Isis as one battle in a global conflict. Merely from a strategic viewpoint, the narrow approach lends itself to clear end points, strategic success, and a limited commitment of resources; the broad approach doesn’t.

Cameron’s expansive concept of the terrorist enemy, into which Isis is located, goes hand in hand with an endless conflict that can’t be won.

The broadly defined “war on terror” (defined beyond defeat of the “core al-Qaida” actually responsible for 9/11) has been a strategic failure: Islamic extremists have proliferated since 9/11. The UK should not repeat the mistake of committing to open-ended conflict with a vaguely defined enemy that ultimately blurs the very line between war and peace that the use of force is supposed to uphold.

Isis wants a wider war. Don’t give it to them.