The defence of the realm, as it used to be called, is a strange old piece of British business.

In the week in which it was announced that hundreds of loyal Gurkhas are to be paid off, it was odd, to put it no higher, to hear assorted ministers and brass bewailing the possibility that disloyal Scots might voluntarily quit the defence establishment.

Britain's armed forces are not what they were, and that's a fact. It is a political fact, too, ordained by the three Westminster parties, while Scottish taxpayers go on contributing their share to the latest defence overspend, or to the renewal of a status-symbol weapons system that we don't – and we are quite clear about this – want.

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In a rational world, nevertheless, Scotland's independence could well be construed as the ideal Whitehall cost-cutting measure. Why not? Philip Hammond, the Defence Secretary, might deride the idea that anyone can "break off a little bit" of Britain's armed forces as laughable. Hacking off bits, even large chunks, is currently his job. Lately the UK's "integrated" defence has been dismantled piece by contested piece.

How is the realm defended, in any case? In the past half a century, Britain has spent more than most, in percentages of GDP, in order to "punch above its weight", maintain its reach and make itself useful to the US. Efforts have been expended on three broad areas. None has involved the defence, explicitly, of these islands. None need constitute a burden on any future Scottish Defence Force.

What has Britain been up to? Broadly: Ireland, foreign wars and nuclear fantasies. If there have to be arguments over what can and cannot be afforded, that's where the money has gone. A good part of it was and continues to be Scottish money: when it comes to defence spending, the south of England is demonstrably the subsidy junkie.

Yet Mr Hammond complains, undaunted, that a new base for Trident would cost still more billions, and claims – you have to grant him nerve – that this would somehow become Scotland's problem. Independence negotiations would hinge, says the Defence Minister, on the acceptance of a bill for the removal of a system the Scottish majority has always abhorred. A mask slips: such rhetoric is part of the price we pay.

Fortunes have been spent on missiles whose use is, all agree, unthinkable. That hasn't dimmed enthusiasm for new Trident toys. Still more treasure has gone on foreign adventures ranging from the morally arguable to the utterly fraudulent, generally in the face of Scottish protests. One upshot, entirely predictable, has been the emergence of a bloated security apparatus.

Ireland's Troubles were a trickier case, obviously enough. The indisputable fact is that they were one consequence of Union. Scots might understand the history better than most, but the responsibility for keeping the peace in Northern Ireland lay wholly with the UK. On what basis, save under a UN mandate, would an independent Scotland have had an involvement?

Alex Salmond is right, then, when he claims a two-fold argument in favour of a Scottish Defence Force. We could be rid of "the biggest concentration of nuclear weapons in western Europe", and gain "the right to decide whether or not to participate in international engagements". The First Minister overlooks only one bonus: we would no longer be part of one of the world's biggest arms dealing operations.

The example of the Republic of Ireland (there are plenty of others) is pertinent. That country has managed perfectly well for decades with a modest defence establishment. It has lacked dangerous international "ambitions", but its forces have served the UN with distinction. Why is that unthinkable for Scotland?

History is part of the answer. Scotland's absorption into Britain was advanced first and foremost by military means, through the regiments. The Union was cemented, and traditions were founded, as "rebellious Scots" were transformed – consciously, deliberately – into the foot soldiers of empire. It's one of the "bits" Mr Hammond did not mention, but it remains a tie that is hard to break, not least for the MoD. What about the rest?

If the comical Defence Secretary is intent on spreading anxiety, Mr Salmond is too busy being glib. He finds a handy "template" in the Coalition's strategic defence and security review and says, in effect, "That'll do". One air base, one navy base, one mobile brigade: simple.

He and his colleagues then demolish Mr Hammond's post-imperial arrogance over weapons of mass destruction. An independent Scotland thus emerges, laudably enough, as a modestly armed little country minding its own business, ready to help out in the world, but not to intrude.

So how do you get rid of Trident, exactly? Or rather, to be strictly fair, when do you get rid of Trident? If Mr Hammond's remarks are any guide, the MoD is in a mood to be obstructive. All of Whitehall's plans for nuclear weapons depend on Faslane, after all. The base is an extremely valuable asset. What happens if a Scottish Government serves a notice to quit and the Royal Navy fails to heed the request?

Mr Salmond no doubt intends to forestall that possibility during independence negotiations. It takes no stretch of the imagination, though, to see any number of technical objections from Whitehall. Mr Hammond has already claimed that it would take "many years" to identify an alternative site and construct a base. How many years?

You needn't doubt Mr Salmond's sincere abhorrence of WMD to grasp the point. Anyone who thinks a vote for independence means the instant removal of Trident is being naive. London will not suspend its nuclear capability because of a mere vote. What follows? The chances are that Faslane will be leased, for a decade or two.

A similar fog of mock war hangs over SNP policies towards Nato. The public stance is well-established: the party will have no truck with a nuclear-armed alliance that has developed the habit of attacking places far from the North Atlantic. The trouble is that Mr Salmond's colleagues are not wholly united in this view. The further trouble is that Scotland remains of strategic importance, like it or not, for London and its allies.

Presuming that he wins in a referendum, it becomes a question of Mr Salmond's appetite for horse-trading. He already intends to keep the pound, and with it the shackles forged by the Bank of England. He has no interest in alienating the monarchy, capstone of the British state. He cannot – whatever he might say – remove nuclear submarines with the stroke of a pen. Will he then withstand intense pressure, at home and abroad, to maintain Scotland's place in the West's mutual defence alliance?

The point, perhaps, is that defence was supposed to be one of the easier parts of the independence process, a divvying up and a fond adieu. Those who thought about it at all tended to worry about jobs, as well they might: Faslane alone employs 6500 people directly, with a further 4000 involved in supplying its needs. But Nationalist politicians have been slow, publicly at least, to acknowledge Scotland's status as a vital UK defence interest, and slower to explain what that means.

Mr Hammond's was just the first salvo. London will fight harder for Trident and Nato than for most things. A dirty little political war is coming.