Image caption Clive Russell and Douglas Russell portray Robert the Bruce and his brother Edward in the two-part series

It's been compared to Scotland's Vietnam - a 14th century attempt to unify the Celtic nations, invade Ireland and try to drive the English out of it.

It briefly saw Robert the Bruce's brother made king of Ireland and featured a 300-ship invasion of Ulster, sieges and battles that marked turning points in Irish history.

Yet the war, which began in 1315, lasted for three years and could easily have been the source material for Game of Thrones, has been largely forgotten.

A new two-part BBC documentary drama series aims to address that fact.

War of The Three Kings is narrated by actor Brian Cox and features Clive Russell as Robert and Douglas Russell as his brother Edward.

Ambitious brother

It combines stunning locations, dramatic re-enactments and CGI, to recreate the harsh and often violent medieval world of 700 years ago and features a variety of academics and experts, among them Dr Gavin Hughes of Trinity College Dublin.

Dr Hughes said there were different theories about the motive for the invasion, including that Robert the Bruce wanted his ambitious brother and heir Edward out of the way.

Image caption A harsh and often violent environment is recreated in the documentary drama

"There's an idea that they send him across just basically because Robert wants rid of him and thinks we'll make him king of Ireland and that'll keep him happy - there's possibly a little bit to that," he said.

"But there's also the idea that if a Scottish army establish themselves in Ulster and take Carrickfergus, then the whole seaboard route is theirs and they can cut off supplies to Carlisle where Robert is having a huge siege."

Celtic unity

The invasion also owed something to the idea of Celtic unity.

"You get to the end of the 13th century when Kind Edward I is on the throne and he's hammering the Welsh and he's also hammering the Scots with Wallace and that kind of thing," Dr Hughes said.

"And the interesting thing is this whole sense of Celtic consciousness begins to develop outside England."

Whatever, the motives for the invasion, Dr Hughes said people in Ireland took sides for different reasons - some to settle old scores, while some were motivated by who they thought would ultimately win.

"It ends up being an almighty mess. There are Irish fighting Irish, there are English fighting English, on the Scottish side of the water there are Scots fighting against Bruce because they support the Plantagenets as well," he said.

"It's all intrigue and a lot of warfare and of course the warfare comes right at the wrong time for a campaigning season, because it comes smack bang in the middle of a famine as well and that's what really does for the Scots army in Ireland."

Image caption Battlefield archaeologist Dr Tony Pollard, left, and Dr Gavin Hughes, Trinity College Dublin - two of the expert contributors to War of The Three Kings

The high point of the Scots in Ireland was 1316 when they cut a swathe through Louth and almost down to Dublin.

Dr Hughes said they could have taken Dublin in 1317, "Dublin was wide open, but for some reason they head off to Limerick.

"After that they're sort of dead in the water because they're not really getting resupplied as much as they should be," he said.

Body cut into four

The key battle was fought in October 1318 at Faughart outside Dundalk and the outcome sounds reminiscent of an episode of Game of Thrones.

"Massive, massive battle and he [Edward the Bruce] is killed there," Dr Hughes said.

"His body is apparently cut into four and sent to all four corners of Ireland as a warning and his head is placed in a casket in salt to be taken to King Edward II [of England].

"It's probably the biggest medieval battle in Ireland, certainly since Clontarf. You think people know about Clontarf, they don't know about Faughart."

There were also a number of key battles in Ulster, which the Scots retained control of for three years.

"The main battle is the siege of Carrickfergus, the first battle is one that very few people know about called Mounthill, overlooking Larne," Dr Hughes said.

"The Anglo Norman lords of Ulster gathered on this very large hill from where you could see Carrickfergus and Larne and the landing points where the Scottish army came in in May 1315 - 300 ships coming towards east Ulster, it must have been a real powerful sight.

"I kind of think it as the Scots' moment of D-Day in medieval terms, because they've got to have a foothold in Ulster and they have to break out towards Carrickfergus and if the Anglo Normans had beaten them at Mounthill, then the chances are the invasion would have been dead in the water.

"As it turns out the Scots absolutely decimate the Anglo Norman lords."

So why is so little known about such a pivotal moment in the history of Ireland and the rest of the British Isles?

Image caption Edward the Bruce was the biggest loser of the invasion, but was briefly king of Ireland

"Both sides seem to have deliberately forgotten about it," Dr Hughes said.

"It's the classic thing that history is written by the winners and Edward Bruce was a very big loser, but he did die as king of Ireland.

"It's something that people didn't want to remember and it probably wasn't politically wise for them to remember it in the aftermath of the invasion.

"Seven hundred years on, it's nice to see it actually remembered, because it was a really important military campaign."

The first of the two-part series is available here.