‘Imperial, war and museum – the three worst words in the English language.” Imperial War Museum director Diane Lees is quoting one of her predecessors, Alan Borg, who reckoned he ran the museum with the most forbidding title in the world. I had asked her why, since she is overseeing the biggest overhaul at the museum for a generation, she didn’t drop the word “imperial”; maybe even “war”. Anyone for the Museum of Global Conflict? Or maybe Battle Space?

“We’ve had that conversation several times,” she admits, “and I’m fascinated by the archive of the trustee discussions on that very matter.” The museum’s peculiar title has, it seems, long been up for debate, but all three words have survived the latest Big Think. “War,” says Lees by way of explanation, “is always going to be fascinating to a certain section of the audience, and our job is to broaden that audience. The imperial bit is accepted as a historical fact as opposed to a political agenda. Andmuseums have changed their image – more people now visit museums than attend football matches. The Imperial War Museum is an internationally recognised brand, and any brand consultant will say, ‘Throw that out at your peril.'” Battle Space will have to wait.

The great naval guns outside the museum, which is housed in the oldBethlem Hospital in Southwark, south London, have also survived. They are the first thing you see as you walk into the park that encloses the museum – municipal land over which she has no control, Lees tells me, disowning the down-at-heel cafe near the guns that will sit oddly alongside the three snazzy new cafes being installed as part of the £40m refurb.

The museum is still a bit of a bomb site when I visit, even though the reopening on 19 July after a year-long refit is only a couple of weeks away. I am allowed to peer at the redesigned atrium from a balcony, a dustsheet being peeled back to reveal a decluttered space with just nine iconic military objects. The Mark 1 Spitfire and V1 and V2 rockets that have always been there have now been joined by a Harrier jump jet, thecar mangled in a suicide bombing in Baghdad in 2007 that Jeremy Dellerturned into an art installation, and a Reuters Land Rover – symbols of wars in which the UK has been busily engaged over the past 20 years. The Sopwith Camel biplane that used to hang in the atrium has gone into the enlarged first world war galleries, and many of the other objects that used to be in the entrance area have been rehoused or pensioned off.

Most of the £40m has been spent on opening out the atrium and remodelling the first world war galleries – to coincide with this summer’s centenary – but this is only the first phase of redevelopment and refurbs of the floors devoted to the second world war and the world since 1945 are also planned. First world war, second world war, the rest – that is the tripartite narrative adopted by the museum, which, born in 1917 in the shadow of the battle of the Somme, remains essentially a record of the British empire’s struggle in the two global conflicts. Nigel Steel, the museum’s principal historian, makes no apology for this regimented portrait of the past century.

“The Somme was such an enormous event for the nation that it was felt we had to make a record of it,” he says. “The main mover was Sir Alfred Mond, commissioner of works and a cabinet member. When the second world war breaks out, our terms of reference immediately get expanded to cover that as well. Then in 1953, when the Korean war is ended with an armistice, it becomes all of the major wars of the 20th century that Britain and the Commonwealth have been involved in.”

Steel thinks it is inevitable that in this centenary year and with the new galleries it will look as if the first world war is unduly dominant. The million-plus annual visitors and all those curriculum-following school groups mostly come for the sections devoted to the two world wars, and there is a danger that everything else – grouped together on the third level – could get lost. But Steel insists it’s all there if visitors are willing to look: Korea, Malaya, the end of empire, the age of the Bomb, the Falklands, Northern Ireland, Iraq, Afghanistan. Quite how you can manage to absorb all that in a visit that averages a couple of hours is, though, a moot point. My guess is visitors will be more inclined to dwell in the new “virtual trench” than head up to the area devoted to asymmetric warfare.

Why, though, do we need a museum dedicated to British military endeavour? “We are now at a point where the first world war has fallen from living memory,” says James Taylor, lead curator on the new first world war galleries, “and it’s up to us to give a shape and context to that conflict, which the distance of time allows us to do. We mustn’t use the objects as relics but make them speak.”

The museum tries not to express one view of history – the mantra, repeated by everyone I talk to, is that its displays are designed to encourage visitors to think for themselves. There is, for instance, no desire to blame Germany for the first world war, nor to glorify the allies’ eventual success. Lees sticks to Mond’s original prospectus that the museum should record “toil and sacrifice”. But it is not possible to be entirely opinion-free, and in the great debate sparked off by the centenary over whether the first world war was worth fighting, it is clear where the museum stands.

“The key thing is that Britain makes a conscious choice to go to war when it cannot be said that it is directly threatened, or at least that’s how we might see it today,” says Taylor. But, he insists, that view relies on hindsight. “At the time, Asquith and his cabinet are thinking, ‘Our strategic aims are threatened.’ A lot of what people understand about the war has been received through poetry, or Birdsong, or even something like Downton Abbey. It leaves them with a very different view from the one people had at the time. The key words that come up [among visitors] are ‘senseless’ and ‘futile’. We try to show how people saw the war at the time. Visitors always ask, ‘With all those casualties, why didn’t they stop?’ But, if you look at the evidence from the time, that’s the very reason people can’t stop – you need a justification for such terrible loss and that can only be victory.”

The museum’s raison d’etre is to use contemporary objects and testimonies to recreate the world as those living through wars saw it – or see it, in the case of the soldiers in Afghanistan from whom it is currently collecting oral histories. What is on display is only a tiny part of the museum’s total collection, which includes 32,000 objects, 10,000 sets of private papers and 750,000 photographs. Borg, who oversaw the last major refurbishment when he was director in the 80s, makes the point that the museum’s key work is unseen. “Although the millions of people who go through the galleries are important,” he says, “and we hope they go out thinking, it’s the documents and film departments which are really vital. The important thing is to have enough money to keep the academic and collecting sides of the museum operating.”

Borg, who later went on to run the V&A, tells me that during his tenure from 1982 to 1995 he sought to humanise the museum. “It seemed to me that, provided we didn’t have another immediate world conflict, the role of the museum would have to change as there wouldn’t be veterans around,” he says, “and the way in which we attempted to change it was to get across the idea that the museum was actually about people. We had lots of large bits of hardware – tanks, aeroplanes and so on – but the real subject is people.” This commitment to telling individual stories, he says, chimed with the new passion among the public for unearthing family history, which helped to stop the museum looking like an anachronism.

Another reason it remains relevant can be found at the very top of the building, in a gallery devoted to contemporary art. The show chosen for the reopening – running alongside an exhibition of first world war art – is a series of films and photographs by Mark Neville, who spent 10 weeks embedded with British troops in Helmand province in Afghanistan in 2010. If anyone doubts that the museum is willing to give space to voices that question war, they should march up here, because, with his slow-motion films shot from a passing tank of bemused or accusing Afghan faces and photographs of desperately youthful British soldiers, Neville is quietly subversive. “What the two films I shot in Bolan market reveal is the deeply complex relationship between us as an occupying force and the locals,” he tells me. “You can see that, although some people are smiling, there’s a sense of something else going on behind the eyes.” He also filmed locals against backdrops of photographs taken by the Irishman John Burke during the second Anglo-Afghan war of the 1870s – a simple and powerful comment on the cyclical nature of war.

“What’s important about Mark’s work is how different it is from the journalistic images you see from the war in Afghanistan,” says Kathleen Palmer, the museum’s head of art. “It will ask questions and won’t give easy answers, which is how it should be for contemporary conflicts. Artistic responses to war are an important part of the museum’s programming. Artists look at things differently. We can talk about facts in lots of other exhibitions, but this reaches people in a different way.”

Palmer had a hand in acquiring Deller’s bomb-mangled car for the museum, and believes the fact it has been chosen as one of the key objects on display in the atrium is significant. “It’s not a sculpture, it’s an object in the collection now and the artist is very clear about that,” she says. “The project we did with Jeremy [in 2010] made us look at how the large objects in the gallery were displayed. Bringing in something that had been destroyed talked about the consequences of war. As well as having big objects that are fully restored and show what weapons would have been like at the time, it is a key moment for us to have something that embodied the destruction of war right at the heart of the museum.”

What, I ask Palmer, is the function of the museum – to record war, interpret war, or warn against its baleful consequences? “It’s all those things,” she says. “It’s about telling the story, but it’s about telling many stories, not just giving one overview. We offer a range of voices from a conflict and allow people to make up their own minds.” She insists it will never be triumphalist. “It’s about what people actually experienced, and trying to get that across.”

In that sense, it is pleasingly British and understated. Borg tells me that when the museum was conceived at the end of the first world war, there was a plan to give it a vast space in Whitehall. Instead, it began life as an open-air exhibition in Trafalgar Square, and was then shunted into cramped quarters in South Kensington, before finding its Southwark home in 1936. The proposed Whitehall museum might have fallen prey to glorification and political manipulation; here, among the park-dwellers of Southwark, it can offer a less statist take on war.

Mrs Thatcher, when she was prime minister, wanted a bit more glorification, especially in the section devoted to the Falklands. Borg tells me a story about his first encounter with her. “She came round for a small dinner to launch our appeal in the 80s and I had to take her through the old galleries. My predecessor had done an exhibition called War, and it opened with a series of billboards – supposedly newspaper headlines but all, in fact, quotations from people about war. She immediately found one she didn’t like – Benjamin Franklin’s ‘There was never a good war or a bad peace.’ ‘That’s wrong,’ she said. ‘You should take that down.’ I said, ‘Prime Minister, you have to see that they’re meant to be taking different views.’ I rapidly scanned the wall for something I thought she would like, and said, ‘Compare it to that one.’ I forget what the line was, but I pointed to it, and then my eyes slipped to the attribution underneath. It said, ‘Mao Zedong.’ I thought, ‘We are in for a bad evening.'”

One day, some future Big Think really will have to consign the Imperial War Museum to history. There will come a point where the first world war ceases to look like the start of everything and becomes part of a continuum. How much longer can the war on terror and the struggles of the present century be treated as addenda to the great wars of the first half of the 20th century? Something will have to give. But, for the moment, we should celebrate the museum’s reopening, and the British way of embracing difference – and diffidence. We may be unduly keen on going to war, but at least we haven’t built a monument to our martial spirit. Which other country, after all, would have housed its military museum in a former asylum? War and the madness of war.