Mary Dejevsky: What is the role of our armed forces if it's not to defend us?

G4S appears to have convinced doubters that all was fine, until suddenly it was not

When a judge ruled that residents of Fred Wigg Tower in east London would have to put up and shut up over the siting of a missile battery on their roof, justifications included the resonant, almost archaic, "defence of the realm". You can argue until kingdom come whether the Olympics should present a security threat of that order – ministers keep reiterating that they are about sport not security – but after the Black September attack in Munich 40 years ago, and the temptation for the aggrieved to seek spectaculars to rival 9/11, it does not seem unreasonable for any government to treat the Games as a potential target. It would rather be remiss if it did not.

Which is why the Home Office announcement yesterday, two weeks before the Opening Ceremony, that 3,500 troops were to be drafted in to help with security came as – one hesitates to write the words – such a bombshell. Where the missiles in east London, plus HMS Ocean in the Thames, Typhoon fighter jets at Northolt and Puma helicopters in Ilford, have been ridiculed in some quarters as overkill, the belated discovery that a private-security firm has failed to train sufficient staff looks an awful lot like underkill.

No wonder the Home Secretary, Theresa May, was on less than sparkling form in the Commons when she tried to defend the decision to bring in troops. It was not the decision that needed explaining so much as the inordinate trust invested by ministers and the Olympics organisers in the private-security company, G4S. May's evasiveness on the matter of penalties was perhaps the most unsatisfactory of all her unsatisfactory answers. What on earth were the terms of that contract – a contract whose fulfilment was bound to have a bearing on our national reputation – if it could be broken at such a late stage and to such a degree? Heads, as is said in equivalent circumstances, should roll, not least at G4S, which appears to have convinced doubters that everything was all fine, until suddenly it was not.

In the end, though, this sorry episode might do this Government and its successors a favour. For the healthier response to the announcement that the army is to be brought in to staff checkpoints is not "how disgraceful", but "what took them so long?" Why was London's Olympic security seen as a suitable case for delegation to a private company rather than a fundamental aspect, during this summer at least, of our national security?

To put it another way, as an Olympic sportsperson or a spectator, would you rather have your ID and your bag checked by someone selected, trained and disciplined to the exacting standards of the British military, or someone rushed through a mainly box-ticking exercise to make up the numbers and save a commercial contract? Would you prefer the safety of the Olympic Park to rely on temporary agency workers, however carefully selected and vetted, or people who are accustomed to taking orders and owe their allegiance to the Crown? The absurdity of the question is its own answer.

So why was a private-security company, albeit a huge one with a track record of government contracts, preferred to the military when the plans for London 2012 were drawn up? One reason might be that the military had more pressing, more important, perhaps more "professional" things to do, such as keeping the peace in Sierra Leone, preparing the exit from Iraq and fighting the Taliban in Afghanistan. As successive home secretaries liked to say with reference to terrorism: "We're fighting over there so we don't have to fight over here."

That is one way of looking at it. But if our troops are so busy in foreign parts that they do not have the numbers to "defend the realm" back here, or do not see it as a priority, then something is very wrong. Few doubt that soldiers, by and large, like to do what they are trained for and recent wars, while widely unpopular, have in fact benefited recruitment. But if economics mean we cannot do it all, surely the home front should come first?

There may also be a more complicated reason. An endearing, and entirely positive, characteristic of the British is an aversion to militarisation. Of course, we like pageantry and parades, but not the serious stuff, such as sharpshooters on the roof (or missiles). Years ago, there was public criticism when the guards at Buckingham Palace and elsewhere had their old rifles replaced with scary modern automatics. And the show of hardware, including tanks, at Heathrow in the run-up to the Iraq war, prompted indignation in Parliament and an inquest into who had given the order. The Government was, therefore, concerned that the Olympics – so far as possible – did not look like a military exercise.

But there was surely another reason, too, for hiring G4S: the fetish of the last government and this one with trying to slim the State by "outsourcing" anything that others are thought able to do. That anything now includes all sorts of security functions. It includes building and staffing new prisons; transporting prisoners to and from court; apprehending and removing illegal migrants. The creation of the UK Border Agency to staff and enforce frontier controls – the title placing it at arm's length from government – was part of the same trend.

Yet there are some functions that are absolutely crucial to the State, and ensuring security – of citizens or the frontier, or the once-in-a lifetime London Olympics – is surely one of them. And if this is not a role for the armed forces, you have to ask what is: fighting latter-day colonial wars?

As announced two weeks ago, the armed forces face severe cuts, in staffing as well as spending. Given the inability of G4S to fulfil its Olympic contract, and the proven inadequacy of the UK Border Agency to control the frontiers, might not a solution be to reorientate the military towards the home front. National security is a prime function of the State, and in this day and age it takes many forms. A smart, mode rn, uniformed force, trained to handle state-of-the-art weapons when required, but also to scan passports, track down illegal migrants and – in this instance – check bags at Olympic Park, would be the acceptable, and trusted, face of British national security. With the fad for privatisation and our misguided foreign adventures, this is something we have not had for a very long time. Unintentionally, but beyond doubt, G4S has shown where private security stops and the responsibility of the State has to start.

m.dejevsky@independent.co.uk