Once, it was housed in a darkened, secretive underground bunker.
Now, Permanent Joint Headquarters (PJHQ) staff have emerged blinking into the light, in a new HQ building that has been nicknamed - only half-jokingly - Cell Block H, for the endless staircases and corridors in the modern new building.
Set up in 1996, this headquarters brought together the Royal Navy, Army and RAF to plan and execute the UK's military operations around the world, although PJHQ still remains less well-known than its US counterpart, Centcom.
It is rare for journalists to gain access, and if the essence of a successful military campaign is secrecy, PJHQ has been doing a good job.
Few outside the world of defence really know what goes on inside the iron gates - and once inside, security remains tight.
Even the entrance pods for staff are reminiscent of the spy series Spooks, while all mobile phones and cameras must be handed in, even those belonging to PJHQ staff.
So what exactly does PJHQ do?
Its head, the chief of joint operations, Air Marshal Sir Stuart Peach, terms it the "getting on with things" headquarters, with its staff effectively on operational tour in the UK.
More than 600 personnel look after the 18,000 UK servicemen and women deployed on operations in 26 locations from Afghanistan to Iraq and Kosovo.
"We have been slightly in the shadows," he admits. "But what we do 24 hours a day is vital to every form of operation."
PJHQ was set up after operations in the early 1990s in the Gulf, and elsewhere, showed the UK needed one headquarters to pull operations together on a permanent basis, liaising between Whitehall and commanders on the front lines to turn government policy into military action overseas, from fighting wars to keeping the peace.
The headquarters sits - sometimes uncomfortably, some say - between the strategic level at the Ministry of Defence in London, and the Armed Forces operating on the ground, though those here insist it does not duplicate effort in London or elsewhere.
"We work around the clock to support or command operations around the world as required," says Sir Stuart.
"We have a 24/7 feed of information and turn that into direction, command and guidance, and discussion.
"Every day things crop up which are new and unexpected, from the tragedy of casualties which we feel deeply here, through to turning a policy choice in London into a practical direction."
The nerve centre of PJHQ is the operations room, several large screens with live video feeds dominating the space.
It is divided up into global sectors, from Afghanistan to Iraq. One sign over a desk reads, perhaps ambitiously, "rest of the world".
The duty operations controller is Lt Cdr Fiona Dobie from the Royal Navy.
She is the initial point of contact for deployed UK troops 24 hours a day, 365 days a year, working as part of a bigger team - and she and her colleagues are the first to hear the news of deaths in Afghanistan before it is passed on to the families of the dead and wounded in the UK.
Even at this distance, it is no easy job.
"It is sometimes difficult when you get the phone calls that you don't wish to receive, about serious injuries and deaths, but there is a variety, a huge amount of variety in the job that we do," she says.
On the Afghanistan desk, Lt Col Alex Macintosh has been the "ops" team leader for more than two years, and has also often had to deal with the deaths of colleagues fighting in Helmand.
"Any death is desperately sad - and we feel it here. There is a gloom that descends on this place, but we immediately have to get on and deal with it quickly.
"Everyone here knows someone who is out there, so of course when someone you know is either wounded or killed, it hurts.
"But we are professionals and the process is there to ensure that everyone is informed in the right order."
Brigadier John Lorimer DSO has experienced PJHQ from both sides, working previously as brigade commander in Iraq and Afghanistan and now at PJHQ itself.
"We provide that glue between the MoD and our deployed forces across the world.
"One of the key things we have to do is respond to the unexpected, and it can get pretty frantic.
"We can be both a buffer and a conduit, keeping the MoD informed - but also wanting to make sure our people out in theatre have all the information they need to do their jobs," he said.
While we film, the screens play video footage from a drone filming over Afghanistan that has already been cleared for security.
PJHQ may have let the cameras in at last, but this headquarters is certainly not letting down its guard.