The defence secretary, Gavin Williamson, has told the Daily Mail: “I do not believe that any terrorist, whether they come from this country or any other, should ever be allowed back into this country … we should … destroy and eliminate that threat.” When asked to clarify how he intends to deal with British citizens who fought for Isis, he confirmed that his default position was to kill them.

British Isis fighters should be hunted down and killed, says defence secretary Read more

Williamson’s comments are at best naive – the hubris of someone insufficiently experienced for their position. But at worst they are morally, legally and practically wrong. They imply a desire for extrajudicial killing to form part of Britain’s security policy. This is so radical a departure from all that we should value, and the way that we should conduct ourselves, that it is hard even to countenance.

Sometimes the use of force, including lethal force, is necessary. But I also know first-hand – from my time serving in both Iraq and Afghanistan – the importance of rules governing it. What soldiers term their “moral compass”.

When on military operations, your proximity to death and violence – and to those who do not acknowledge the rule of law – both challenges your belief in the importance of such laws and reinforces the need for them. Adhering to such laws can at times be difficult: courageous restraint requires extraordinary moral resilience.

But how we utilise force is what differentiates us from our opponents. Soldiers know they lose their legitimacy when they sink to the level of the terrorists they face, and that such legitimacy is maintained through the rule of law. If soldiers understand that, secretaries of state should too.

Soldiers are taught that for force to have legitimacy it must: distinguish between those who pose a threat and those who do not; be proportional in terms of what you are trying to achieve; and be borne out of the need to protect life and prevent further suffering, not a desire for retribution.

The policy Williamson suggests would ignore these rules, ignore the law of armed conflict they underpin – and replace them with a blanket policy more akin to murder than any concept of justice.

Gavin Williamson is wrong to think that the way we answer hate is to change what Britain stands for

I know that the UK and our armed forces have sometimes broken these laws. It is right that the International Criminal Court investigate claims that British soldiers committed war crimes against detainees in Iraq. The UK is not, nor should it ever be, above the law.

But even these alleged crimes are fundamentally different from what the defence secretary seems to be advocating. Such a policy would not only break the Geneva convention and UK law, but would undermine our society. We could no longer turn to our troops and require of them, in often the most difficult of circumstances, to obey the law of armed conflict. We could no longer turn to other nations and ask them to end extrajudicial killing. And we could no longer ask the British public to trust in the rule of law, when we, the lawmakers, are not prepared to do so ourselves.

Williamson was right to condemn British citizens who had gone abroad to join terrorist organisations. He may also have been right that “they hate everything that Britain stands for, hate our values, hate that Britain is a beacon to the world of democracy and tolerance”. But he was wrong to think that the way we answer that hate, and the threat that it poses, is to change what Britain stands for, change our values and undermine our standing as a beacon for democracy and tolerance.

Tough choices have to be made. But politicians will only regain the faith and trust of the public – something we have lost in recent years – if we can demonstrate an ability to understand the complexity of the issues we face, take informed decisions that act in the best interests of our country, and not just pursue either a tabloid headline or increased standing among political colleagues. In this instance, that means that killing terrorists should not be our default setting. Where we can arrest them and bring them to trial we should do so; lethal force may remain necessary where we can’t.

Our answer to the threat that terrorism brings must be unequivocal, but it must also be legal. Because the rule of law is key both to the society we are defending and how we defend it, and if our response to terrorism is to destroy human rights and the rule of law, the terrorists have already won.

• Dan Jarvis is a Labour MP and a former British army major who served in Iraq and Afghanistan and sits on the joint committee on national security strategy