When Britain’s political leaders decide to make an issue of defence, they sometimes boast that the United Kingdom remains the world’s fifth biggest military power. Whether you find the claim reassuring or depressing is, of course, a matter of taste.

In terms of cash, it seems to be true enough. Early this year, in its global survey, The Military Balance 2015, the International Institute for Strategic Studies (IISS) confirmed the UK had held its spot in an interesting league. In 2014, austerity or no austerity, $61.8 billion was spent in our names.

These things are relative. Standing fifth in a league dominated by the US, still spending close to four and a half times the outlay of China, doesn’t exactly make you a contender. By IISS reckoning, the Americans had a budget of $581bn; the Chinese $129.4bn; Saudi Arabia $80.8bn; and the Russians $70bn.

What, though, is the UK doing keeping company with superpowers and despotic economic giants? The French got by with $53.1bn; Japan $47.7bn; and Germany $43.9bn. You could be tempted to make the kind of point Whitehall finds annoying and puerile: if we only spent what the Germans spend, George Osborne wouldn’t have to wage war on the poor.

The defence establishment will have to live with its irritation. The fact is that the UK’s “standing in the world” is a very expensive commodity for a country supposedly on its uppers. The military budget is less a matter of defence than of ambition, pretensions, and projected power. It has also been a function, down the decades, of that special relationship with the US.

Plenty of analysts will tell you that cyber attacks are the biggest current threat to our defence. Regardless, the UK is spending £6bn (at least) on a couple of aircraft carriers. That’s before the vessels can be furnished with their American-supplied F35 Joint Strike Fighters at £70 million a machine. That’s how “fifth biggest” is bought.

Status and perceived standing dictate defence policy. “Britain’s role” these days takes us to war without much consideration of war’s purpose. At the time of writing, Westminster lobby correspondents are arguing over whether David Cameron will or will not stage a Commons vote on airstrikes against Islamic State (IS) in Syria. What we know for sure is that the Foreign Affairs Committee has given the Prime Minister a warning: don’t bother until you have a “coherent international strategy”.

Some might be surprised by that. Who attempts to spend a few more tens of millions from our defence billions without a plan that counts as coherent? The committee, chaired by the Tory MP Crispin Blunt, is polite in its language, but damning in its conclusions.

There is (this might sound familiar) “legal ambiguity”. With that comes the risk of “reputational damage”, catastrophic for diplomacy. There’s the “complication” of Russian bombing in the region. There’s deep uncertainty over the possible political aftermath. And there is an answer to a $61bn question: the UK’s involvement, says the committee, would have only a “marginal effect”.

In these affairs, Washington likes to have its Brits around, but no one is under many illusions about their usefulness. One school of thought within the UK’s own military has long argued, in fact, that the country’s ability to join these campaigns is hampered utterly by the uses to which the defence budget is put. We spend a lot that could be spent on other things. In purely military terms, we spend badly.

Never mind the carriers. Of the budget total, 6 per cent already goes on Trident. The Minister for Defence Procurement, Philip Dunne, has recently put the “acquisition cost” of four new submarines at £25bn. That’s just the start. The latest estimate of “in-service costs” between 2028 and 2060 - Mr Blunt’s office did the sums – gives a total of £167bn. Hence the marginal relevance of airstrikes on Syria. The UK lacks the capacity to do more.

Mr Cameron has tried, now and then, to explain himself. As with the Iraq debacle, his justifications make airstrikes sound like a panacea. Somehow they would begin to solve all problems – IS, the Assad regime, refugees, Islamisation – at once. The fact the US-led Operation Inherent Resolve has had what the IISS calls “mixed results” since it was launched in August 2014 is ignored. In the Prime Minister’s telling, a handful of UK jets and drones would make all the difference.

Patently, the Foreign Affairs Committee is not impressed by that optimism. The MPs are not alone in that. As with Trident, however, many Conservatives and more than a few Labour members are hypnotised by the myth of a UK role. A vast defence budget becomes its own justification. The world’s fifth biggest military power must do its bit, even when the Prime Minister has no coherent idea of what that bit might be.

Famously, Tony Blair gave an insight into establishment thinking in his 2007 book A Journey. It is still worth quoting. Of Trident, he said: “The expense is huge, and the utility in a post-Cold War world is less in terms of deterrence, and non-existent in terms of military use. Spend the money on more helicopters, airlift, and anti-terror equipment? Not a daft notion. In the situations in which British forces would most likely to be called upon to fight, it was pretty clear what mattered most.”

Mr Blair also admitted that “it is frankly inconceivable we would use our nuclear deterrent alone, without the US”, but decided that giving it up was “too big a downgrading of our status as a nation, and in an uncertain world, too big a risk for our defence”. How the loss of an unusable weapon presented a risk was never obvious, but the former Prime Minister’s other point was relevant. Trident and the other big toys hinder rather than help with things - “anti-terror equipment” - that Mr Blair, like Mr Cameron, would call essential.

Scottish Labour’s Trident vote is one small step towards sanity. Asking why the UK spends quite so many billions of dollars (the appropriate currency) more than richer nations without achieving military relevance might be another. France has its cherished nuclear weapons and still spends $8.7bn less than the UK. And the French are not the best available model.

Where Syria is concerned, having less of a bang for our buck is no bad thing. The committee is right about that. The Prime Minister wants to bomb for no better reason, it seems, than that bombing is what a modern UK prime minister does. If he has been restrained, that’s all to the good. There is no “case for war”.

The government claim that bombing a war zone would lead to “stability” is madness even by the standards of the defence establishment. Instead, perhaps Mr Osborne will now give us an estimate of the money we could be saving.