IN the autumn of 2004, just weeks into her job as director of the newly created National Theatre of Scotland (NTS), Vicky Featherstone's eye was caught by a newspaper article. It concerned the Black Watch, then on its second tour of Iraq. More particularly, it concerned the political row that had erupted over a US request - subsequently granted despite objections in parliament - for the regiment to be moved from Basra to Camp Dogwood, a base south of Baghdad in the so-called "Triangle of Death".

As it turned out, the name was apt. In November 2004, in two separate incidents, the regiment lost four soldiers and an interpreter. A month later, a different sort of loss: the long-expected announcement by General Sir Mike Jackson that the Black Watch, along with five other Scottish regiments, was to be absorbed into a new cadre called the Royal Regiment of Scotland. Nearly 300 years of tradition looked set for the scrapheap.

As the weeks rolled by, Featherstone's interest deepened. On the face of it, the story had nothing to do with theatre but she could see that its essence was pure drama. She knew too that the powerful societal and historical currents merging in it were exactly the sort of forces the National Theatre of Scotland had been founded to explore.

Fast forward 18 months. It's the early summer of 2006 and I'm talking to Dunfermline-born playwright Gregory Burke, who tells me what happened next. "Vicky assigned me to keep an eye on it, knowing that everybody from Fife has a connection with the Black Watch. She said there might be a piece coming from it."

A piece did come from it: Black Watch, written by Burke and based on informal interviews he conducted in late 2005 with former Black Watch soldiers who had served in Iraq. It was due to be performed at the 2006 Fringe and would mark the NTS's first festival show. Burke said it wasn't political and that there was no agenda beyond representing the voices of the men who were living, fighting and dying in Iraq.

"I just want to get people into the theatre to watch the play," he continued. "It's about saying this is how it is, this was the experience for this group of people. I was interested in it because it relates to me. It relates to Fife. It relates to people in my family who have been in the Black Watch. It relates to the history of the regiment, to something that's part of Scotland."

AT that point the play was an unknown quantity, although expectation was high. Nobody, however, was quite prepared for the popular and critical response that met its opening on August 1. The venue was a drill hall in the Old Town, specially kitted out to mimic the seating at the Tattoo just a few hundred yards away on the Castle Esplanade. By the end of the play's three-week run, it had become the most celebrated theatre space in Edinburgh and was the launch-pad for an incredible international journey.

To date, that journey has included a tour of Scotland in March and April of last year, followed by high-profile runs in New York and Los Angeles in the autumn, and performances in Australia and New Zealand, the only country it has toured to so far that doesn't have troops in the Iraqi or Afghan war zones.

In Dingwall, BBC Scotland filmed the performance and screened both it and a documentary about the play on consecutive nights during the 2007 Fringe. Featured in the documentary was Fifer David Ironside, a former Black Watch soldier who had had a breakdown as a result of his experiences in Iraq and whose story had formed the basis for one of the characters in the play. He had turned up to the Dingwall show with his girlfriend. For director John Tiffany, it was Ironside's testimony, caught on camera, that proved the single most moving moment of the whole Black Watch experience. He recalls how Ironside shook as he told how it was only after seeing the play that his girlfriend could realise what he had been through.

"We were able to articulate someone's experience in a way that they couldn't explain to their girlfriend," says Tiffany. "He said he felt that she now understood why he was a f***-up."

In June 2007, the play became a genuine cultural artefact when the NTS was invited to mount three special performances to mark the official opening of the Scottish parliament. The announcement was made by a pleased-as-punch Alex Salmond at a hastily convened press conference in St Andrews House in Edinburgh. Sharing the stage with him was the culture minister, Linda Fabiani. Neither politician had seen the play at that point. Also on the dais was a slightly sheepish-looking Vicky Featherstone, perhaps wondering where all this was going to end.

The event itself was a starry occasion though even it was outgunned by the celebrities queuing for tickets to the American shows a few months later. Energised by a rave review in The New York Times and intrigued by the subject matter, New York theatregoers headed en masse to St Ann's Warehouse. The 23-night run was a sell-out but among those who did manage to get tickets were Lou Reed, Rupert Murdoch, Stephen Sondheim and actress Frances McDormand. Cate Blanchett, Hugh Jackman and Sam Neill caught the Sydney shows.

The big question is: why? What is it about this play that has spoken so directly to audiences not just in Scotland but in Los Angeles, New York, Wellington and Perth? Is it the mixture of music, dance, folk songs and pipers? Is it the fact that it is a simple story about war, told in the vernacular of the men who are fighting it?

"It was Tennessee Williams who said that to be truly universal you have to be truly specific, so it's only through that specificity that we've been able to connect with something much bigger," says Tiffany. "As a piece of theatre, Black Watch was never designed to go beyond that drill hall in Edinburgh. But I suppose in hindsight the Middle East is the story of our time, and that's the case wherever you are in the world."

FOR his part, Burke is no less baffled by the Black Watch phenomenon now than he was a year ago when we last spoke. He recalls an interview with The New York Times, which went some way to explaining the appeal of the play, at least to the Americans.

"In the east coast liberal press, they have this dichotomy. They don't want the war but they don't want to be seen to be not supporting the troops, and I think that's what Black Watch chimed with. The Americans are a patriotic nation, even those on the left."

As 2007 closed, the Black Watch juggernaut showed no signs of slowing. Newsweek listed it in its five best plays of the year; New York Magazine went further, calling it the theatrical event of the year, as did the New York Observer. The Los Angeles Times did not demur from the general consensus that this had been a historic piece of theatre from Scotland's fledgling national theatre.

So what next? Like The Cheviot, The Stag And The Black, Black Oil before it, the power of Black Watch lies in its distillation of a particular moment in time into a vibrant piece of theatre. It can't roll on forever, nor should it.

"I don't see how you can revive it," says Burke. "I think at a certain point you have to say enough's enough."

John Tiffany agrees. "All theatre has to be urgent. By that I mean it has to be about now and relevant to now. So I'm happy that Black Watch will have a short shelf life. In fact it's had a longer shelf life than I'd ever have imagined." But, he adds: "What it's done is emboldened us, given us courage to see that this is what we're good at."

The use-by date hasn't been reached yet, however. There is still some unfinished business for Black Watch, some quarters in which its power has yet to be felt. In the summer it will tour to England for the first time, which means London's chattering classes will finally get to see what all the fuss is about. There will be a Welsh premiere in Ebbw Vale in May. Then it will be performed in Toronto and Virginia, returning to New York in October for another run at St Ann's Warehouse.

Before all that, it will be performed in the Fife town of Glenrothes. After its wanderings, does Gregory Burke feel the play is finally coming home?

"Glenrothes is a major recruiting town for the Black Watch and a couple of boys from there got killed as well, so I think it's great that it's playing there," he says. "But I don't feel personally like it's coming home. It's weird. I feel like it belongs to Scotland now."

Black Watch is at Rothes Hall, Glenrothes, from Thursday-April 4 and at the SECC, Glasgow, from April 11-18