In March, the Russian ambassador to Denmark told Copenhagen that, by electing to deploy American missiles that can shoot down Russian nuclear ones, its navy had become a legitimate target for a Russian nuclear strike, should it ever come to war.

Though it caused outrage in diplomatic circles – and renewed fears that Vladimir Putin’s next target lies in the Baltic – once you understand the concept of deterrence, the Russians were only stating the obvious: “Since you have a weapon that can kill our weapon, we must kill your weapon before we fire ours.”

Here we are having a debate about our own nuclear deterrent: or rather, half a debate. At present, the argument focuses on whether to scrap or renew Trident; or to replace it with cruise missiles, which fly more slowly and carry fewer warheads.

But some in the defence world believe the whole debate is deluded. Trident, in its current form, was designed to deliver “minimum deterrence” – that is, using as little force as possible to threaten Russia with “unacceptable loss”. The method is to maintain at least one submarine continually at sea, armed with up to 12 missiles, each capable of dropping eight warheads on to enemy cities.

But few people understand that the UK’s nuclear deterrent is a system – not a technology – and one that must constantly evolve as the threat changes.

For a nuclear-armed submarine to pose a credible threat, it must avoid being tracked by Russian submarines. To do that it must get out of the River Clyde and, once at sea, be protected by aircraft trying to spot the Russian subs, plus an undersea surveillance system whose sensors are scattered across the ocean bed. On top of that, those controlling Trident must engage in a data-crunching battle with the Russian navy, whereby each side uses predictive modelling to guess where the other’s subs will be.

For informed critics and supporters of Trident, the difficulties start here. In 2010, David Cameron cancelled a programme to upgrade the Nimrod surveillance aircraft tasked to look for the Russian subs. The UK’s are now protected only by helicopters and ships.

The new problem is that Russian attack submarines are parked close to Britain’s shore with increased frequency. Unlike the Russian bombers now ranging over Scotland and Cornwall, these submarines cannot be seen, and the MoD will not comment on submarine operations. But the circumstantial evidence is there.

In January, Britain had to call in two US surveillance planes to help track a Russian sub. This week, a UK fishing boat snagged what it believed was a Russian submarine just off the Isle of Man. HMS Talent, an attack submarine whose job is to hunt its Russian counterparts, had its turret damaged by “floating ice” – which was the old, cold war euphemism for collision with a Russian vessel.

So, the first awkward question is: if Trident plus Nimrod was a “minimum” defence option against a Russia that had given up aggression, how is Trident minus Nimrod still an adequate system, given this increased Russian submarine activity?

The second, much bigger question is: what is the renewed Trident actually supposed to deter?

During the cold war, deterrence was designed to make the Soviet Union reconsider invading western Europe with conventional forces. But with Vladimir Putin’s sudden turn to proxy warfare in Ukraine, and with Russia blocking UN action against a Syrian regime that has openly used chemical warfare, you are dealing with a completely different situation.

Putin is KGB-trained, and many of Russia’s old military doctrines still operate – but we are not dealing with like for like. There is no Politburo and no Marxism. The Soviets derived their version of blitzkrieg from an idea about economic warfare pioneered by Mikhail Tukhachevsky: assault the enemy throughout the depth of his formation. They were playing a long, relatively predictable game.

With Putin, nobody is sure what the game even is. Nothing Ukraine’s allies did deterred Russia from seizing first Crimea and now eastern Ukraine. Sanctions had no effect and various Nato countries are reduced to bilateral aid to Ukraine, since the alliance itself is split.

Plus, the politics of Europe are now different. Today the threat is to Baltic states towards whom there is scant solidarity among the populations of the original Nato countries. Meanwhile Putin’s Russia has far more support among the British financial elite than the Soviet Union ever had in the British labour movement.

General Sir Richard Shirreff, Nato’s former deputy supreme commander in Europe, told me: “What is completely missing is an understanding in the national debate that we face a game-changer in what Putin has done. If he did decide to try and have a crack at the Baltic states, people really need to understand that that means war.”

But they don’t – and in part that is due to the lack of coherence to the public debate.

There is no published Ministry of Defence evaluation as to whether the current Trident system is sufficient to deter the new, unpredictable Putin – either from invading Estonia or nuking the Danish navy. All published UK research, for and against Trident, is pre-2014 – and assumes the threat of war with Russia is close to zero.

The unpalatable truth – for those who believe in nuclear deterrence – may be that four new submarines are not enough. All the things touted as alternatives to the current Trident system – cruise missiles, free-fall bombs and static silos – might be needed on top of it. Without a clear, public assessment of the new threat, nobody knows what the new minimum deterrent really is, or if it can deter at all.

For those who oppose nuclear deterrence as a concept, the challenge is to spell out an alternative doctrine to deal with the Russian threat.

But in the election debates, we’re getting neither of the above. We’re getting instead, from all sides, a contested shopping list, whose relevance to the worsening global security situation is not immediately obvious.

Paul Mason is Channel 4 News economics editor. Watch his report on Britain’s defence options.