Those people seduced by the SNP's obsession with abolishing Britain's nuclear deterrent should perhaps use Google to find the Budapest Memorandum of December 1994.

They would see there a document representing the deal struck when Ukraine, holding the world's third largest nuclear weapons stockpile, agreed to give them up in return for solemn security assurances from Russia, the US and the UK.

These countries, with France and China as well, promised to a) respect Ukrainian independence and sovereignty in its existing borders, b) to refrain from the threat or the use of force against Ukraine, and c) to refrain from using economic pressure on Ukraine in order to influence its politics. Don't these promises look good in the light of the carnage we see on our TVs every night?

Yet that is what Ukraine got in return for unilaterally disarming. Some bargain. And it is legitimate to ask this; would Crimea have been grabbed and Eastern Ukraine occupied if the Ukrainians had kept some of their nukes?

But yet the Nationalists have said they will end Britain's nuclear deterrent without even getting the promises made to Kiev.

In the face of all the manifest threats we face today and the unknown, unknowable ones we may face in the next 50 years what sort of folly would it be to permanently dismantle our deterrent this year?

President Putin certainly has no intention of doing so. He has ominously reminded the world publicly Russia has nuclear weapons, and what's more he is ordering more and bigger ones. Two new types of submarine-launched ballistic missiles, a new class of ballistic missile submarines, a new type of ballistic missile, a new bomber and long-range cruise missiles - all additions to his nuclear arsenal.

China too is adding to its stockpile, as are - and for their own reasons - India and Pakistan. Iran is on the way there and then we have the crazy and very unstable men in North Korea. These countries don't share the Nationalist's naïve and simplistic adventurism. As the others do all this, our Nationalists want to leave us - and future generations, completely exposed in a dangerous and unpredictable world.

I started my own political career, as many people know, carrying a Ban the Bomb banner. By giving up the Bomb I believed that we would give a lead and start a benign chain reaction. I was wrong. As Defence Secretary of this country I was responsible for the biggest ever reductions in our nuclear weapon capacity and it led to no copycut reductions elsewhere.

And yet it's not even as if the Nationalist unilateral disarmers know what they are talking about. There are constant references to a new Trident missile system when what is proposed are new submarines to replace the existing ones which come to the end of their life. The missiles remain exactly the same, the British designed warheads only slightly changed. New submarines, not a new system, is the reality but why change a good slogan with the truth.

And the cost? With very Nationalist speech the figure grows and exaggerates like Pinoccio's nose and is expected to pay for every pricey promise. Of course it may look a big sum - but the system has to exist for the next 50 years and when you spread it over that period the costs are pretty small for the security they provide. Over the years it will account for only 5 - 6 per cent of the defence budget. The total bill is the equivalent of a few weeks spending on the NHS.

The same obsessives constantly tell us that Trident will not stop terrorism or cyber attacks or the like. Agreed; but it is not supposed to do so. Other deterrents are needed for these modern threats. Nuclear deterrence is to stop any potential aggressor from thinking he or she can win a conventional war or launch a nuclear attack or even blackmail us. And it has done that successfully for the last sixty years.

Nobody can predict what challenges and threats we will face in the next five, never mind 50, years. Given what we see today it would be an impossibly knowledgeable or a profoundly reckless person who would dare say that we will never be threatened as a country or as an ally over these generations. Those who advocate the abandonment of our deterrent ask others yet unborn to face terrible risks.

The real world out there is a dangerous place, as the people of Ukraine are sadly finding out. It is no place for those who would bargain away our long-term security and safety.

Lord Robertson of Port Ellen has been Secretary General of Nato, Secretary of State for Defence and Shadow Secretary of State for Scotland.