In 1998 the UK government did perhaps the best thing it has done to date: it conducted a Strategic Defence Review (SDR) which was foreign policy, not resource led. Instead of defence policy being cobbled together in response to whatever scraps the Treasury affords the MoD, new Labour very sensibly intended that it should be the physical manifestation of our foreign policy objectives. And those objectives were made nothing if not apparent. For when, a few years hence Jack Straw and Tony Blair attend international conferences, the response of their less well endowed peers will be, 'are you just pleased to see us, or are those a pair of bulging aircraft carriers you've got there?'

As Professor Lawrence Freedman (one of the external participants in the SDR) explained, the review led to a 'consensus within the defence establishment behind the need for new, large aircraft carriers. The service chiefs had seen little point in buying replacement aircraft carriers for HMS Invincible, and her two sister ships, unless they could carry many more aircraft. This would require ships as much as twice the size of the Invincible'. But understanding why something should be done, and even making a pledge to do that very thing are not, as this government daily displays, guarantees that those things will be done. The new carriers have been pledged during a time of plenty – it's not too hard to see a future, when the economy's not quite so hot, where choices might have to be made. Such as between hospitals in Rotherham and e.g. big ticket, easily cut items of defence procurement. Therefore in addition to asking, what will these lovely new status symbols do, and why, we need to ask, will they ever be built?

There can be little doubt that the Royal Navy performs miracles with the inadequate 20,000 ton Invincibles. Their deficiencies illuminate the last time Labour was responsible for big decisions like building carriers. In the mid sixties the Admiralty had drawn up plans for a new class of fleet carriers. The CVA-01 would have been about 50,000 tons, and equipped with  fixed wing aircraft. They would have been expensive, but what scuppered them was not cost. Instead it was the combination of an inadequate strategic rationale, and the arrival of an arrogant intellectual.

The latter was Denis Healey. The former can be summed up as, the exigencies of alliance defence. Cold War Britain saw NATO as the alpha and omega of defence. 'The West' was the idea we were fighting for, but, if we were going to fight, the only place we were set on doing so was central Europe. This meant that the continental commitment (i.e. the army and RAF in Germany) always trumped naval arguments. When the RAF argued it alone could discharge the aerial contribution to NATO the carriers were left financially exposed. Their sole justification would have been if Britain intended to 'bear burdens' outside the NATO area.

This possibility was killed off with Healey's 1966 Defence White Paper:

only one type of operation exists for which carriers and carrier borne aircraft would be indispensable; that is the landing or withdrawal of troops against sophisticated opposition outside the range of land-based air cover.

Thus, as the Defence Secretary put it in the ensuing parliamentary debate, 'British carriers are not necessary for operations in the Atlantic, Mediterranean or Middle East'. No new carriers were commissioned, and those in service were to be phased out by the end of the seventies.

That should have been that, but the brute cunning of the senior service is never to be underestimated. One NATO role only the RN could perform was waging anti-submarine warfare against the Soviets. Ship-borne helicopters were the principal weapons system. To carry a substantial amount of helicopters what inescapably was needed was a fairly substantial vessel. Since proposing carriers to ministers was akin to swearing in church, these were called 'through-deck cruisers'. By the time their design was completed they had swollen to 20,000 tons (more the displacement of a Great War battleship than a cruiser) and the Heath government commissioned the first Invincible in 1973. However, all the while the Admiralty was conscious of what the new V/STOL (vertical/short take off and landing i.e. aircraft that could take off from small carriers) technology could mean. In the Harrier they had found a way of saving the carrier. Thanks to one of those eleventh-hour moments of British amateur brilliance, this possibility was triumphantly vindicated when Lt. Commander DR Taylor invented the 'ski-jump'. Which allowed Sea Harriers to get airborne at significantly reduced speed – the inevitable consequence of operating off small platforms.

The heroics of 1982 – when, thanks entirely to the surviving carriers, the Navy pulled off an operation it was never meant to be capable of fighting – ensured all three Invincibles were built. Despite their not in truth having any NATO-centric justification. In effect, we maintained a marginal carrier capacity, without quite believing in the doctrinal reasons for doing so. Now that the Cold War has been dead a good decade, the love that dared not speak its name – having juicy big carriers – has erupted out of the Strategic Defence Review. We want them big, and we know why we want them. It is, in short, the [George] Robertson doctrine: 'in the post Cold War world, we must be prepared to go to the crisis, rather than have the crisis come to us'. This is not merely expression of a military truism (of course to fight, you have to go to the fight), rather it is enunciation of a very disturbing foreign policy. Engagement is the polite term; in reality, under Blair we are set to play global special constable to America's world policeman, and the new carriers are his moral truncheons.

So, apart from, is this all just fine sounding Blairite waffle, will a future government actually cough up truck loads of dosh to pay for the world's largest, er, manhood magnifiers? What can the carriers do and to whom? Well, as the Americans show, if they're big enough they project power. What one does with power is a moral consideration and therefore beyond my scope, but power is what carriers deliver. How large then will ours be? Defence Procurement Minister Lady Symons trills that they 'are likely to be the biggest warships ever built in Britain'. I'm sorry to be too train-spotterish here, but that would make them about 50,000 tons, just about the minimum necessary to avoid all the drawbacks of the Invincibles. The government in fact has set a ceiling of 40,000 tons. Size matters because that determines the size of aircraft a carrier can take. Which in turn determines their individual capability. The whole point of this exercise being to acquire carriers which carry fixed-wing aircraft, not the less capable V/STOL airframes (e.g. the Sea Harrier).

Not that there is an aircraft yet. This purchase will cost more than both carriers. The choice is between either an Anglo-American project currently in development, or, crossing our fingers and hoping that a naval version of Eurofighter can be magicked into existence. This latter option would inescapably entail a ship of at least 50,000 tons. The 50,000 ton mark has serious implications as to where it would go i.e. it will be a tight squeeze into any of the remaining naval dockyards. All of this though is to ignore the most basic question of all, where will it be built? Yards which could handle a vessel of this scale e.g. Harland & Wolff are far from certain to be around in five years time. Then there is the issue (ruled out for no reason other than cost) of why shouldn't they, like the American ships, be nuclear powered? Which would, after all, tie in exactly with their supposed rationale: being there to facilitate 'anytime, anywhere' action. There's a strong case for saying that the sole determinant of size should be that ours end up longer and wider than those of the French.

If they are built, what are the tactical implications of the government's decision that, in order to afford them, we'll cut back on escort ships? Most of these dilemmas (given that having large carriers is an intrinsically pleasurable and desirable activity) could have been forestalled if a visionary Defence Secretary had bought ex-Soviet kit at bargain prices, and handed them over to UK shipyards for rebuilding. Unfortunately the man that that vision would have been required from was pre-penitent Michael Portillo.

Even if Blair's British beauties are ultimately built – and especially if they're the seriously expensive vessels they need to be in order to be worth the effort – what government is likely ever to have the courage to place them in harm's way? For (unlike the three-strong Invincible class) there are only going to be two of them. The government claims that 'this reduction in hull numbers is to be achieved through modern build and support techniques which will dispense with the need for long refit periods and will allow required ability to be achieved from only two hulls'. Regardless of how diverting or otherwise one finds this fantasy, it ignores that the test of these carriers (and the strategy behind them) is how they would perform during war. One might sink. The possibility that they might be used to fight in the first place is infinitely more credible if there are three, not two.

There's much wrong with defence policy. The closest the British army should be to the Rhine is Kent. Another carcass clung tightly to is the RAF. One may as well say that any given piece of equipment – fax machines for instance – should have it's own service. In principle there's nothing wrong with carriers; now more than ever a maritime strategy is the one for Britain. But if the principles these vastly expensive weapons serve amounts to little more than armed enforcement of global counselling sessions we, and the rest of the world, would be a lot better off without them.