Thomas Sutcliffe's Notebook: Oh what a lovely war museum!

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If ever there was a public building that appeared braced for self-defence it's surely the Imperial War Museum. In front of its classical portico, two massive 15-inch naval guns lift in gentle elevation, covering the institution against the possibility of a surprise attack from the City of London. Read the plaque by their massive breeches and you discover that the first time these engines of imperial power projection saw action was in 1920, when the idea that a gunboat might coerce good behaviour was still just about afloat, but their intimidating heft – they could throw three-quarters of a ton of explosive some 18 miles inland – has been redundant for decades now.

Just across the path sits a different kind of obsolescence – an upright wedge of the Berlin Wall, bright with graffiti and acquired for the museum in 1991 – when the idea that war itself might be considering retirement was still rising gently into the air, on a thermal of post-Cold War optimism.

Both objects have a certain poignancy in these uncertain days – souvenirs of times when uncertainty at least had an optimistic edge to it. But they're just a taster for the contradictions that wait in the museum itself. On a midweek morning, the fact that the world is pregnant with conflict seems to have thinned attendance a little – but it's done little to quell the excitement of the school groups, scurrying around with clipboards beneath the bric-a-brac of old wars.

They jostle through the basement corridors, darting in and out of the exhibition spaces and making it abundantly clear that it is curiosity they feel here, rather than anything more solemn. "EastEnders!" shouts a small boy, when his attention is directed to a large aerial photograph of a German bomber over the London docks; only after closer inspection does he realise that it isn't Walford under threat. "Wicked!" breathes a teenager as he pushes into a section devoted to the bombing campaign against Germany – and a word that has had much darker applications in recent weeks is briefly recovered for childish enthusiasm. "My grandad flew a Lancaster," his friend says, proudly staking a claim to a bit of inherited valour as they inspect a cut-away model of the aircraft in question.

But for anyone a little more preoccupied with current affairs the museum is alive with glancing ironies – little jolts of unintended significance. It's there, in the section devoted to contemporary wars, where the interactive screen identifies Afghanistan as a troublespot and concludes its potted history of invasion and civil war with a judiciously open-ended phrase: "Conflict continues". Round the corner you can find a modest stretch of wall (perhaps four foot in all) devoted to Terrorism, this threat to global stability represented by a cheap suitcase, a stack of cardboard boxes and a short passage of descriptive text which – understandably perhaps – treats the subject as a dangerous nuisance, not a casus belli. It's also difficult not to feel the false superiority of hindsight when you stand before the large exhibition case devoted to the Gulf War, with its memorabilia and its video footage of President Bush announcing that "our military objectives are met". As the "long thin war" gets under way that short, broad one – the cause of so much agonising at the time – can be seen as almost enviable in its clarity.

The accidental dialogue between past events and present is occasionally uncomfortable. A diorama of a devastated German city offers an older echo of New York's ground zero – rubble spiked with twisted steel – and, as evidence of some visitors' indifference, a dusty peach pit surreally out of scale with the carefully modelled destruction. Most queasily reconfigured of all the exhibits though is the Blitz Experience. This, warns a sign outside, is considered "unsuitable for children under five, people of a nervous disposition or those who dislike confined spaces". They can add another category now, because anyone uneasy about the simulation of human vulnerability will find this recreation of civilian terror an odd exception to the general delicacy about anything that might scrape on the raw wound of 11 September.

Visitors are first ushered into a darkened shelter, lit only by an artificial oil lamp, where they sit in darkness as a carefully modulated concussion pounds around them. The benches jerk pneumatically as an explosion comes close and then, after the all-clear sounds, they're ushered out into a devastated city street. In the gloom, a pram wheel turns in the rubble, in a touch of servo-assisted Spielbergian pathos. When the gas works go up there's a gasp of "Oh my God!" from a speaker near by – the recorded exclamation a pallid echo of more recent, and more wrenching cries of horror. A pale theatrical smoke drifts across the street before you encounter the restorative glow of a Womens Royal Voluntary Service tea stall. It was "the lady with the tea what sorted us out in the end" says a voice dense with cockney pluck.

Outside, the attendant on duty says he's had no complaints about the exhibit from visitors, American or otherwise. And although the Museum briefly closed down an exhibition of props and costumes from the film Pearl Harbor, out of sensitivity for its visitors' feelings, the Blitz Experience has run without a break, offering catastrophe every 10 minutes to those who want something more than artefacts. "It's just a personal view," says the attendant, "but you can't fudge history." He's right, of course. The Blitz can't be unmade by the attacks on America, even if they make its artificiality painfully obvious. Besides, the Imperial War Museum is not a casually bellicose place, nor one that brushes over the deep melancholy of war.

But at a time when the fear of war – and of civilian casualties – is as high as it's been for years, there's still something oddly analgesic about this place. A kind of escapism accompanies the depressing reminder of the murderous efficiency of contemporary war. It takes a while for me to work out what it is until, wandering around a reconstructed Korean war dugout, I finally pin it down. It is the consolation of the past tense – emphasised on every side by these labelled objects, with their implicit promise that they have no further use but contemplation.

Staring at a bullet-damaged helmet or a Second World War machine gun, you're aware of a ghost exhibit for which, in time, the museum will have to find space. One day there will be other uniforms here, other obsolete weapons and scraps of painted metal doing their best to summon an intensity of experience no longer really recoverable. Outside, history might be speeding up beneath our feet, like a treadmill running out of control, but in here, mercifully, it's standing still. This may be one of the few places in London, in fact, where it's possible to imagine that this strangely formless war is already over.