This blog was also published in The Guardian's Comment is Free

Britain's military capacity is likely to shrink following the soon to be released Strategic Defence and Security Review. But many of the most feared changes may be desirable, or even necessary, if we are to fulfil some of our most basic moral obligations. The trick will be to combine change required by current fiscal constraints with an honest assessment of morality, and integrate both into a new strategic vision of what our forces should be and how they should be used.

Thinking clearly about morality and the use of force is hard because we have developed strong mental habits of avoiding these issues, or of thinking about them in highly distorted ways. Morality is a broad canvas, but at its core stand a small group of rights that almost everyone agrees have critical importance: the right not to be tortured, raped or sexually abused, the right not to have one's liberty or property arbitrarily removed, and most centrally the right to life.

Although we have duties to aid and rescue others, our most important moral duties, by far, are to not violate the core rights of others. Shooting my neighbour in the head is much worse than failing to protect him from an equivalent harm, even if I have contractual or other duties of care towards him.

This simple fact places a very high burden of justification on the use of force. An enemy can become liable to defensive force if he is morally responsible for an imminent attack on our rights, and if our defensive action is not substantially more harmful than the attack we seek to avert, and if there is no less morally costly way of achieving the same result. Otherwise he retains his right to life and intentionally killing him is a form of murder. In practice this means that the use of military force the mass intentional killing of other human beings can only be justified in response to an imminent large scale attack on society.

But many of our costliest weapons programmes have very little to do with homeland defence understood in this way. The two planned aircraft carriers and the Joint Strike Fighters they will carry are for projecting force to far corners of the globe. Boosters say that the ability to field and use such expeditionary forces enables the UK to "punch above our weight", sustaining a global influence beyond our modest means. But it is important to realise that from a moral perspective these are just not the kinds of goods that can justify the use of force. No matter how familiar and historically resonant, the idea that we can parlay a readiness to use lethal force for influence and national advantage cannot be morally sustained.

The biggest ticket item of all is the replacement of Britain's aging Trident nuclear weapons system, and it is here that the case for change is most compelling. In fact rarely are moral, economic, and strategic imperatives in such close alignment. Nuclear weapons by their nature are indiscriminate, that is to say rights violating, and almost all independent assessments agree that they make little real contribution to Britain's foreseeable security needs in the 21st century. The government's commitment to Trident seems to be based on concerns of 'status' that Tony Blair candidly reveals in his memoirs to have been uppermost in his own thinking about Trident, and on the vacuous argument that the bomb should be retained as a stupendously costly hedge against future uncertainty. We should co-ordinate our actions so as to generate maximum leverage on the critical non-proliferation and disarmament processes, but dismantling our weapons of mass destruction should be an avowed policy of this government. Would choosing such an explicitly defensive posture doom the UK to the status of a strategic pygmy? Not necessarily. Switzerland and the Nordic nations have effectively used soft power to sustain wealth and significant global influence without costly and morally problematic exercises in force projection.

There is a caveat to all of this. Not violating core rights is our first and most solemn duty. But we do also have a duty to aid the victims of unjust harm at the hands of others. This idea has been formalised in the emerging norm of Responsibility to Protect, and although not entirely free of problems there are clear cases, such as Rwanda in 1994, when a failure to intervene against mass atrocity is itself a grave moral wrong. If we are to be in a position to do this, then we must retain some ability to field forces in far corners of the globe.

Yet the capability required for effective humanitarian intervention is often quite different to that possessed by a traditional military, trained and equipped for high intensity warfare. The necessary skill set is often a composite between policing and soldiering, not unlike that of a robust gendarmerie force. If we are serious about our responsibilities to protect those threatened by genocide and atrocity, then this is the kind of expeditionary force we should be developing together with our international, and particularly European, partners. Not coincidentally, developing this composite of police and military skills would be hugely valuable in countering the non-traditional security threats posed by trans-national terrorists and organised crime.

Much of the defence debate has been starved of ethical context. But decisions over defence are moral decisions to their core: what kind of nation do we want to be? How seriously are we willing to take the rights of others? The financial strictures that form the backdrop of this defence review have been seen almost universally as a threat. But they also create a once in a generation opportunity to reconfigure UK forces on a more morally sustainable basis.

This opinion piece reflects the views of the author, and does not necessarily reflect the position of the Oxford Martin School or the University of Oxford. Any errors or omissions are those of the author.