Most Scots support the UK’s nuclear deterrent. One poll in May 2013 showed 51 per cent in favour and 34 per cent against; another in January 2015 showed 46 per cent in favour and 43 per cent against; and a third two months ago showed 43 per cent in favour and 42 per cent against.

Most Scots recognise, then, that while nuclear weapons can’t deter all threats, they continue to deter some. Twenty-five years ago we thought we’d seen the end of the Cold War; now relations between the West and Russia are freezing again. In recent war games the Russians have practised a swift offensive against Poland and the Baltic states, launching nuclear strikes to dissuade Nato from further resistance.

The Church of Scotland, however, objects to the very idea of deterrence. In its 2009 report, The Ethics of Defence, it exhorts us to trust in God instead of placing other people “in a position of fear or threat”. By threatening others rather than seeking to be reconciled with them, a policy of nuclear deterrence is immoral.

This is surely facile. Of course, fear and mistrust are not symptoms of an ideal relationship. In the world as it is, however, persons and states sometimes do unjust things that give others good reasons to fear and mistrust them. In that case, the road to reconciliation doesn’t lie in pretending nothing has happened and holding out the hand of friendship anyway. Rather, it begins with signalling to the wrongdoer he’s done wrong by opposing it and pressing him to think again and change his ways so trust can be restored. It may be true – as I believe it is – that we should always trust God. But it really doesn’t follow we should always trust Vladimir Putin.

Does the nuclear deterrent actually work? Among the reasons for thinking so are these: there’s been no direct military conflict between the major powers since the nuclear era began 70 years ago; there’s been no direct, all-out conflict between two nuclear states; and no nuclear state has ever been invaded.

Some argue other causes have been responsible – the development of international institutions since 1945 and the rise of global trade. But these don’t add up: rising global trade didn’t prevent the outbreak of war in 1914, nor the League of Nations in 1939.

While a majority of Scots want the UK to retain its nuclear deterrent, many of them would like it cheaper than Trident. The evidence, however, is that an alternative wouldn’t be much cheaper and would be less secure.

Within the past three years two independent studies into alternatives to Trident have been published. The first was a government report sponsored by the Liberal Democrats (July 2013); the second, the outcome of a parliamentary commission sponsored by the anti-nuclear organisation Basic (June 2014). Both bodies hoped for a different answer, but the reports concluded that cruise missiles would be less reliable than Trident’s ballistic ones.

Effective deterrence, however, doesn’t depend only on the missiles’ qualities. It also depends on their being so deployed as to be invulnerable to a first strike. Such invulnerability is currently achieved through one submarine being on patrol in highly secret locations at any one time – the so-called Continuous At Sea Deterrence (CASD). Marginal savings could be made by reducing the number of submarines and having them patrol randomly rather than continuously. But this would create periods when no submarine was at sea, during which the nuclear deterrent would be vulnerable to a first strike.

There is no nuclear weapons system and policy of deployment so effective as Trident and CASD, and whose combined credibility as a deterrent is therefore as strong.

Would that nuclear weapons could be disinvented! But they can’t. Would that a single, bold, brave, clean act of unilateral self-purification would so inspire international trust as to stimulate global, multilateral renunciation! But it really wouldn’t.

What remains, then, but despair? Hope remains, but not impatient and reckless. It’s the kind of hope that, as in the past so in the future, the careful management of nuclear deterrence will discourage tyrants from chancing their aggressive arm; that the incremental strengthening of international norms and institutions will bolster trust and relax tension; that more non-nuclear states can be dissuaded from acquiring nuclear weapons altogether; and that the stockpiles of those already armed can be further reduced.

So there’s room for hope in our prudence. But if there’s also prudence in our hope, then, while we needn’t learn to love Trident, we should learn to live with it.

Nigel Biggar is the Regius Professor of Moral and Pastoral Theology at the University of Oxford and author of In Defence of War (2013).