Dr Julian Lewis: I have often had the pleasure of debating this topic with the right hon. Member for Islington North (Jeremy Corbyn), both in and outside the House, but never in either of our wildest dreams or nightmares did we imagine that one day he would end up as leader of the Labour Party. It only goes to show the unpredictability of political developments.

After the Falklands war, opponents of our strategic deterrent often pointed out that our Polaris submarines had done nothing to deter Argentina from invading the islands. However, there never was and never will be any prospect of a democratic Britain threatening to launch our nuclear missiles except in response to the use of mass destruction weapons against us. But just because we would baulk at threatening to launch nuclear weapons except when our very existence was at stake, that does not mean that dictators share our scruples, our values or our sense of self-restraint.

An example from history will do. Following the horror of the poison gas attacks in the first world war, it was widely expected that any future major conflict would involve large-scale aerial bombardments drenching cities and peoples with lethal gases. Why did Hitler not do that? Because Churchill had warned him that British stocks of chemical weapons greatly exceeded his own, and that our retaliation would dwarf anything that Nazi Germany could inflict. Poison gases are not mass destruction weapons, but nerve gases are, and Hitler seriously considered using them against the Allies in 1943. He did not do so because his principal scientist, Otto Ambros, advised him that the Allies had almost certainly invented them too. In fact, we had done no such thing and were horrified to discover the Nazi stocks of Tabun nerve gas at the end of the war. That was a classic example of a dictator being deterred from using a mass destruction weapon by the mistaken belief that we could retaliate in kind when actually we could not. Such examples show in concrete terms why the concept of deterrence is so important in constraining the military options available to dictators and aggressors.

I shall briefly list the five main military arguments in favour of continuing the specific British policy – pursued by successive Labour and Conservative Governments – of maintaining, at all times, a British minimum strategic nuclear retaliatory capacity.

The first military argument is that future military threats and conflicts will be no more predictable than those that engulfed us throughout the 20th century. That is the overriding justification for preserving the armed forces in peacetime as a national insurance policy. No one knows which enemies might confront us between the years 2030 and 2060 – the anticipated lifespan of the Trident successor system – but it is highly probable that at least some of those enemies will be armed with mass destruction weapons.

Mr MacNeil: Will the right hon. Gentleman give way?

Dr Lewis: No, I am sorry. I normally like to take interventions, but I will not, because of the time pressure.

The second argument is that it is not the weapons themselves that we have to fear but the nature of the regimes that possess them. Whereas democracies are generally reluctant to use nuclear weapons against non-nuclear dictatorships – although they did against Japan in 1945 – the reverse is not the case. Let us imagine a non-nuclear Britain in 1982 facing an Argentina in possession of a few tactical nuclear bombs and the means of delivering them. Retaking the islands by conventional means would have been out of the question.

The third argument is that the United Kingdom has traditionally played a more important and decisive role in preserving freedom than other medium-sized states have been able or willing to do. Democratic countries without nuclear weapons have little choice but to declare themselves neutral and hope for the best, or to rely on the nuclear umbrella of powerful allies. The United Kingdom is a nuclear power already, and it is also much harder to defeat by conventional means because of our physical separation from the continent.

The fourth argument is that our prominence as the principal ally of the United States, our strategic geographical position and the fact that we are obviously the junior partner might tempt an aggressor to risk attacking us separately. Given the difficulty of overrunning the United Kingdom with conventional forces, in contrast to our more vulnerable allies, an aggressor could be tempted to use one or more mass destruction weapons against us on the assumption that the United States would not reply on our behalf. Even if that assumption were false, the attacker would find out his terrible mistake when and only when it was too late for all concerned. An independently controlled British nuclear deterrent massively reduces the prospect of such a fatal miscalculation.

The fifth and final military argument is that no quantity of conventional forces can compensate for the military disadvantage that faces a non-nuclear country in a war against a nuclear-armed enemy. The atomic bombing of Japan is especially instructive not only because the Emperor was forced to surrender, but because of the reverse scenario: if Japan had developed atomic bombs and the Allies had not, an invasion of Japan to end the war would have been out of the question. The reason why nuclear weapons deter more reliably than conventional ones, despite the huge destructiveness of conventional warfare, is that nuclear destruction is not only unbearable, but unavoidable once the missiles have been launched. The certainty and scale of the potential retaliation mean that no nuclear aggressor can gamble on success and on escaping unacceptable punishment.

Opponents of our Trident deterrent say that it can never be used. The two thirds of the British people who have endorsed our keeping nuclear weapons as long as other countries have them, and continue to endorse that in poll after poll – as well as in two general elections in the 1980s – are better informed. They understand that Trident is in use every day of the week. Its use lies in its ability to deter other states from credibly threatening us with weapons of mass destruction. Of course, the British nuclear deterrent is not a panacea and is not designed to forestall every kind of threat, such as those from stateless terrorist groups, but the threat that it is designed to counter is so overwhelming that no other form of military capability could manage to avert it.

If the consequence of possessing a lethal weapon is that nobody launches it, while the consequence of not possessing it is that someone who does launches it against us, which is the more moral thing to do – to possess the weapon and avoid anyone being attacked, or to renounce it and lay yourself and your country open to obliteration? If possessing a nuclear system and threatening to launch it in retaliation will avert a conflict in which millions would otherwise die, can it seriously be claimed that the more ethical policy is to renounce the weapon and let the millions meet their fate? Even if one argues that the threat to retaliate is itself immoral, is it as immoral as the failure to forestall so many preventable deaths?

Moral choices are, more often than not, choices to determine the lesser of two evils. The possession of the nuclear deterrent may be unpleasant, but it is an unpleasant necessity, the purpose of which lies not in its ever being fired but in its nature as the ultimate insurance policy against unpredictable, future, existential threats. It is the ultimate stalemate weapon, and in the nuclear age stalemate is the most reliable source of security available to us all.

[NOTE: At the end of this debate, the motion to renew the Trident submarine fleet was carried by a majority of 355.]