Defence is one area where creating a separate Scottish state has precious few upsides and a multitude of downsides.

Not only would it produce little benefit it would also create more uncertainty in a complex and increasingly unstable world and threaten thousands of highly skilled Scottish jobs.

Britain's defence is highly integrated, professional and effective. Our forces are admired and envied throughout the world. They are a class team and encompass a network of capabilities and relationships which, if unpicked, would be a huge loss to us and the wider world. To do so would be an act of irresponsible vandalism with serious global consequences.

And what would the purpose be? To create some mini army dedicated to peacekeeping? To ape, as some in the SNP say, the long-time and historically "neutral" nations such as Ireland, Austria or Finland? Or to pretend that, overnight, we can replicate the defences of similar-sized countries such as Denmark and Norway?

Denmark and Norway, who accept the nuclear umbrella provided by Nato, have developed their defences over many years and are embedded in Nato's collective defence apparatus. They also spend more on defence than the £2.5 billion a year the SNP say they will spend. They should look at the herculean time and effort made by Lithuania and Slovakia for what it means to become serious, small-state defence players.

The start-up costs in Scotland would be enormous. It is not just about amputating some regiments with "Scottish" in their name and re-badging them as the Scottish Army. There's then the Navy. How many frigates and destroyers and auxiliaries do we grab? And the airforce? How many fast jets, transports and AWACs do we target? Also, there is the centre: a new Ministry of Defence, support and command structures and services, and civilian staff.

The latest SNP policy document says that, out of its £2.5bn annual budget, it will build conventional submarines (which the UK gave up 30 years ago), new frigates (they come in at £800 million a piece) and maritime patrol aircraft (so expensive the Coalition Government scrapped the programme and broke up the Nimrods). It simply does not add up.

There is also more to defence than the visible hardware: logistics, intelligence, reconnaissance, special forces, cyber capabilities and much else. These are formidably expensive and complicated capabilities to create and will not be suddenly acquired by simply taking a scalpel to Britain's defence.

Even if the newly separate Scottish state got that far, its size would make redundant most of the defence industrial companies already in Scotland. Because procurement is not subject to EU cross-border tendering, the remaining UK would hardly buy its equipment from a foreign country. Many defence businesses privately say they see this as inevitable. Thousands of highly paid, skilled jobs would, at a stroke, be under threat.

It is however, in relation to Nato, the world's most successful defence alliance, that the champions of the secession of Scotland really undermine their case for a Denmark-type defence policy. Similar-sized countries rely on being part of Nato's collective defence. Territorial defence is no defence against today's threats. Nato's collective shield is crucial to all nations in the alliance.

Of course the Nationalists say they are now in favour of being in Nato. By a hair's breadth conference majority they unconvincingly said they would sign up. But in an alliance which says in its strategic concept: "As long as nuclear weapons exist, Nato will remain a nuclear alliance", the unilateralism of the SNP places an unacceptable condition on Nato. They actually propose to disarm their neighbouring country by removing the deterrent, and thousands of jobs, from Faslane.

Playing politics with defence is a dangerous game, as others before the SNP have found.

The Scottish people deserve some honesty about what the separatists propose for our nation's proper defence and that is more than they have received so far.

Lord Robertson was Secretary General of Nato, UK Defence Secretary and Shadow Secretary of State for Scotland. He is a speaker at the second Enlightening the Constitutional Debate event run by The Royal Society of Edinburgh and the British Academy, on the theme of defence and international relations at the RSE in George Street on May 29. By ticket only, booking via RSE website.