Johann Hari: The best way to improve Britain's national security would be to reject Trident

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A metaphorical mushroom cloud will hang over Westminster this week. On Wednesday, the House of Commons will debate whether Britain should breach the Nuclear Non-Proliferation Treaty (NPT) and buff up our stockpile of weapons of mass destruction for another generation. Already, one government frontbencher has announced he will resign, and a majority of Labour backbenchers in a BBC survey have declared they will rebel.

The Prime Minister has slapped down a challenge to his opponents: "Those who question this decision need to explain why disarmament by the UK would help our security." He's right. So let's show how insisting Britain maintain nearly 200 nuclear bombs on hair-trigger alert floating around this island, each with eight times the explosive power of the bomb that incinerated Hiroshima, makes us less safe.

There are three ways Britain is threatened by nukes from the outside. Nuclear Threat One: A fundamentalist group could smuggle a nuclear weapon into this country and detonate it. A tiny but determined band of people would like to carry out this plan: Osama bin Laden has declared that the acquisition of an "Islamic nuclear bomb" (presumably followed by Islamic radiation poisoning) is "a religious duty," and he tried to buy one himself in the mid-1990s.

The will is there, and the knowledge is not too hard to get. The only (massive) obstacle to these ambitions is getting hold of highly enriched uranium or plutonium. In theory, these essential nuke-ingredients are kept locked away - but there are recorded instances where they have been guarded by men armed only with garden rakes. The US Council of Foreign Relations explains that in the former Soviet Union, "Even basic security arrangements such as fences, doors and padlocks remain inadequate in many locations."

The head of the Democratic Republic of Congo's collapsing nuclear reactor plant was arrested last week for flogging off his enriched uranium to the highest bidder. (Nobody knows how much is missing).

Trident is, of course, useless against this sort of threat. Non-state actors leave you with nowhere to retaliate: after 9/11, would you nuke Hamburg? Besides, jihadists welcome death. For them, mutually assured destruction isn't a deterrent; it's an incentive.

But does Trident make this situation worse? I think it does, for one reason. Every penny we spend on the illusory "safety" of Trident is a penny we are not spending on securing collapsing nuclear facilities around the globe. John Kerry calculated it would cost £8bn to get all the world's enriched uranium and plutonium sealed away. Replenishing Trident will cost at least £20bn. So let's lock up the nuclear materials instead, and spend the £12bn-change on tracking down the maniacs who want to use them.

Nuclear Threat Two: A regional nuclear war could break out somewhere else in the world and trigger a nuclear winter. This is by far the biggest nuclear danger to us, and the least discussed. There are 30,000 nuclear weapons on earth, and the use of barely a dozen could cause irreparable environmental damage. This is not a wildly implausible scenario: only five years ago, Britain had to advise its citizens to leave India and Pakistan because of the real risk of a nuclear war, and it's not hard to imagine a similar situation soon between Iran and Israel.

There is only one route out of this. It is the NPT, created in the 1960s after the world came within inches of consuming itself in fire during the Cuban Missile Crisis. The NPT is based on a simple deal: the existing nuclear powers slowly scale down their nuclear arsenals, in return for the non-nuclear powers agreeing not to tool up.

The renewal of Trident violates this, our last best hope. As Rabinder Singh QC and Professor Christine Chinkin say in a legal opinion, the renewal of Trident "constitutes material breach" of Article VI of the NPT, which pledges "nuclear disarmament" pursued "at an early date".

A global momentum towards disarmament is the best way to sway Iran and other countries in the Middle East from going nuclear. Of course, only the criminally naive believe the deranged anti-Semite Mahmoud Ahmadinejad will wake up the morning after Britain disarms and realise he doesn't need a nuke after all. Gandhian moral suasion has little effect on religious fundamentalists. But we need to be playing a long game here, appealing to the Iranian people themselves. Most estimates suggest it will take a decade for Iran to have a working nuclear weapon, and Ahmadinejad's domestic popularity is already dissolving. Unless the US and Israel bolster him by attacking, he will be gone before he has access to a weapon.

But the problem will remain: the Iranian people will still want a nuclear bomb, with around 80 per cent demanding one in opinion polls. In their situation, it's not hard to see why. They are ringed by nuclear neighbours, and traumatised by the memory of the CIA overthrowing their democratically elected Prime Minister in 1953 and installing a fascistic dictator.

If we want to change this pre-and-post-Ahmadinejad wish for nukes within Iran, we need to change the external situation. In a world that is ramping up its nuclear arsenals, the Iranian people want a weapon of their own. In a world that is steadily decommissioning its nuclear weapons, they probably would rather spend the money on schools and hospitals, like everyone else. Renewing Trident diminishes the chances of that ever happening - and, therefore, our safety.

Nuclear Threat Three: Some as yet unidentified state will one day emerge and threaten us with nuclear annihilation. This is unlikely, but not impossible: in the 1920s, few people saw Nazism on the horizon. But there is a better way to guarantee against this than Trident. It is known as "the Japanese option".

At the moment, Japan has a virtual nuclear arsenal. They have a civil nuclear programme and advanced rocket technology, so they could put together a nuclear weapon in three months if they needed one. Britain could do the same: retain the capacity to assemble a weapon, but not have one day-to-day. By going Japanese, we would simultaneously strengthen the NPT and retain a guarantee against nuclear blackmail.

So yes, Prime Minister, these are three solid ways in which disarmament would make Britain safer. You talk about Trident as an "insurance policy", but as Bruce Kent of the Campaign for Nuclear Disarmament has warned, it is as though you are taking out an insurance policy against subsidence of your house that contributes to that very subsidence. Let's cling to the naive hope that the fall-out is, in the end, only political.