I'M NOT a pacifist. It counts as one of my many flaws, no doubt. I admire anyone who insists on believing a rotten world can be improved. I certainly prefer people who stick with non-violence to the alleged Christians who missed the Q&A on killing, and why thou shalt not (no exemptions, even for saintly prime ministers).

Grubby reality still persuades me, however, to believe countries sometimes have to be defended, and that sometimes resistance in a just cause looks no different to violence for the worst motives. I simply draw the line at those who flaunt their licence to kill. It isn't clever and it is not, ever, inspiring.

I do not, meanwhile, object to the principle of "liberal interventionism". I simply ask to be told the truth about causes and motives when we ride to the rescue. I also live in hope that those in power will some day remember that not all wars are the same war - that some conflicts are worthwhile and some stupid, pointless or unwinnable. Real democracies have to work out the difference.

That's where I stand (or, as an armchair tactician, sit). We need our armed forces. We need them equipped, housed, paid, cared for and spared political nonsense. But do we really need them, as the jargon has it, to "project British power" in any corner of the globe? To put it another way, do we need two enormous aircraft carriers at £2 billion (this week) and 65,000 tonnes apiece?

When finally the two war machines are named after Prince Charles and his mum, we will be allowed a supplementary question to the familiar one about Trident. Something like this. If, as our government repeats tirelessly, the biggest threat to life, liberty and security in the 21st century comes from small groups of fanatics hoping to attack civil aircraft and urban areas, for whom will those giant carriers provide protection?

If the nuclear arsenal is tricky to explain given the lack of states with the means and motives to assault us, where do 80 attack aircraft deployed half a world away come in? Lumbering military giants have a habit of faltering in asymmetric wars.

Current defence policy appears to amount to no more than the statement, true but useless, that you never know. Best be on the safe side. A better thought, never applied in matters of defence procurement, might be: can we afford it? This isn't a joke when your economy is having gearbox trouble and when those who spend the money (our money) on military kit have a record of stunning incompetence.

In this game, figures are wholly unreliable. Inflation in the arms trade obeys unique rules. Crudely put, nothing ever costs less than the experts hoped. But take the rough numbers: £20bn (but call it 40) for Trident; £4bn (who'll give me six?) for the carriers; plus a good deal more for the support and protection vessels those behemoths always require. Then the cost, already well in excess of any guesstimate, for the fighter planes.

Meanwhile, wise men say we could be in Afghanistan for a generation, at a current admitted cost - no-one believes it - of more than £1bn annually, while our prime minister fails to find an escape route from Iraq. It's a lot of beans for the counters. Even if I subscribed to the war-on-terror paranoia, I'd be asking about the number of fanatical bands of killers you could hunt down at that cost.

These issues deserve more than sarcasm from the likes of me. Governments cannot do everything. They can only do their best to keep us safe after weighing up real and potential risks, and estimating the chances of potential future dangers. If they say the jihadi menace tops every possible menace list, I would want better proof than I have heard lately. But I might, just might, allow the benefit of the doubt. Then I would want to know how you stop a Tube bombing with an aircraft carrier.

One possibility of one variety of threat is being used, it seems to me, to excuse the old, silly, vainglorious pretence of global influence. Those two carriers mean only that someone still believes in the idea of Britain as a "blue-water" power, throwing our weight around the distant oceans while polishing our rented Trident warheads just to show we still mean business. It should all go down well, I feel, with any country given to wondering who we think we are.

There is a complication, of course. Or rather, there are 5000 complications. Scottish (and British ) shipbuilding is not what it was. It relies on a dripfeed of income from the killing trade to maintain its traditions, keep families going, offer apprenticeships and secure something called a knowledge base. They say the carriers guarantee 5000 jobs. Meanwhile, the chances of a British government providing £4bn to preserve the fishing fleet or the merchant marine appear to be remote. So shouldn't I, as a Scot, when times are hard, just lay off?

I've been asked that one before. It strikes me still as misconceived. The job of building those carriers - a considerable co-operative achievement, they say - involves a logic we cannot ignore, in Govan or in Fife. The purpose of those floating airfields is to kill people. We all hope, supposedly, that it will never come to that. But are we to earn our corn as a nation from the assumption that some day, somewhere, strangers will have to die to guarantee our apprenticeships?

The arms trade always corrupts. That's why a previous prime minister developed an aversion to inquiries and fraud investigations. It makes a kind of superficial sense to funnel state aid into the guns and bombs business - useful industries are less fortunate, of course - but such blessings are deceptive, even when real jobs are at stake. As an economic entity an aircraft carrier is a nullity. It doesn't raise crops or educate children, or do anything for the balance of payments. It is, in fact, a form of creative accounting with taxpayers' money.

Political conservatives who lecture us on market forces overlook such points when the flags are waving. The cynics among them recall that America never went broke by going to war (a theory under some stress at the moment). But warfare and the real world are confused only rarely. I call that revealing. In fact, you could almost put it to music. Just what is war good for, exactly?

It is not good for Scotland, I'd suggest. Last week, we were all invited to "celebrate" £4bn worth of good fortune for the people and communities who build ships. No-one asked why the Clyde and the Forth can assemble a pair of impressively big vessels, at the leading edges of marine technology and design, but cannot otherwise make a living. "National security", some would say. So in what sense would our security be enhanced by the Queen Liz and Prince Chuck?

Most people harbour a patriotic impulse or two. Definitions will vary, but generally the argument is manageable. Ask those same people, however, why their public-sector pay rise cannot approach the inflation rate while their government performs the full orchestral version of Elvis Costello's Shipbuilding and they might have some doubts. Yet for some adventures, it seems, money is always available.

There is a final problem with very big, very dangerous war machines. Some foolish people always get the idea our instruments of slaughter are pointed their way. Sometimes they object. Sometimes, as a consequence, no-one's security is enhanced or preserved. Then the decent pacifists have the dubious, redundant privilege of knowing they were right all along.

Once upon a time, the anti-nuclear movement tried to employ history to make a point about febrile nation states and wars. The weapons system has not been invented, they would say, that someone, in some fit of patriotic dementia, has not attempted to use. I still think that was fundamentally true. It's just the idea of having the system stamped "Made in Scotland" that bothers me.