In 780 AD, King Offa of Mercia was at the height of his authority. Prior to his rule, in 750 AD, King Eliseg (immortalized by Eliseg’s Pillar near Llangollen) had swept the Saxons out of the plains of Powys. Offa, in turn, attacked Powys in 778 and 784, and tradition states that he built the dyke, sometime (or throughout) his reign. Prior to this, Aelthelbald, King of Mercia, had built ‘Wat’s Dyke’, which extends from the Severn Valley northwards towards the estuary of the Dee (A History of Wales, John Davies p. 62).
There is a quote from George Borrow, from Wild Wales, that “it was customary for the English to cut off the ears of every Welshman who was found to the east of the dyke, and for the Welsh to hang every Englishman whom they found to the west of it”. This is potentially apocryphal, but indicates the significance of this man-made border between the two countries.
One of the biggest mysteries about Offa’s Dyke, in addition to when it was built, is why? It was a huge undertaking to construct the earthwork, 150 miles in length, up to 65 feet wide and 8 feet high in places, along the entire length of the border between what is now England and Wales. It clearly wasn’t made to keep the Welsh out of England, or to protect the Saxons in Mercia–since it was never defended. Both English and Welsh kingdoms appeared to have a hand in determining where to build it, since it runs to the east of Wat’s Dyke when they are parallel, and in Gwent in particular, leaves lowlands to Wales to the east of natural features it might normally have followed. It was dug, however, “with the displaced soil piled into a bank on the Mercian (eastern) side. Where the earthwork encounters hills, it goes to the west of them, constantly providing an open view from Mercia into Wales.” http://en.wikipedia.org/wiki/Offa’s_Dyke The prevailing opinion to date was that Offa built it as a sign of authority and power–as a means of saying, to a certain extent, ‘after this wall, here be dragons.’
I, personally, like the theory that Offa’s Dyke is a Roman construction:
The Roman historian Eutropius in his book, Historiae Romanae Breviarium, written around 369, mentions the Wall of Severus, a structure built by Septimius Severus who was Roman Emperor between 193 and 211:
Novissimum bellum in Britannia habuit, utque receptas provincias omni securitate muniret, vallum per CXXXIII passuum milia a mari ad mare deduxit. Decessit Eboraci admodum senex, imperii anno sexto decimo, mense tertio. Historiae Romanae Breviarium, viii 19.1
He had his most recent war in Britain, and to fortify the conquered provinces with all security, he built a wall for 133 miles from sea to sea. He died at York, a reasonably old man, in the sixteenth year and third month of his reign.
However, this site, http://www.cpat.org.uk/news/oldnews/offaro.htm explains why this is unlikely. “The evident dislocation of Offa’s Dyke from the currently recognised pattern of early 3rd century military sites in the Welsh borders. This includes the legionary fortresses at Chester in the north and Caerleon in the south, other forts such as those at Leintwardine, Caersws and Forden Gaer, and a road system of which some elements are still in use today as parts of the modern, A5, A39 and A41 routes. The alignment of Offa’s Dyke shows no tangible geographical association or functional integration with this network. Indeed, it is in any case very hard to see what possible purpose such an undertaking could have served in Roman occupied western Britain, especially when the surviving Dyke is actually not a 130 mile complete frontier but is only spread discontinuously over that approximate length with extensive unexplained gaps (80 miles of earthwork are known).”
Furthermore, Ian Bapty, Offa’s Dyke Archaeological Management Officer with CPAT states: “the attribution of the Dyke to Offa by Asser in his late 9th century ‘Life of Alfred’, echoed by the tradition of the ‘Offa’s Dyke’ name itself which can be documented back as far as the 13th century, has been accepted as correct by Anglo-Saxon scholars. ‘Offa’s Dyke is an extraordinary survival from our Anglo-Saxon past’ says Ian Bapty ‘and extraordinary exactly because it is Anglo-Saxon and as such sheds crucial light on a key period of our history when the modern political geography of Britain was beginning to appear. While we can perhaps associate descriptions of the ‘missing’ wall of Severus with somewhat confused and secondarily derived later accounts of Hadrian’s Wall – which was much rebuilt in the time of Severus – we surely cannot backdate Offa’s Dyke to Roman times, and to do so would be to miss the real significance and historical impact of this amazing earthwork’.
‘Ultimately I’d be ready to wager my granny on the fact that Offa’s Dyke is Anglo-Saxon and not Roman!’ says Ian ‘although I’d also have to be say that I’d be keeping granny firmly out of the stakes when it comes to betting on most other aspects of our understanding of the Dyke, including key issues such as exactly why it was built, how it was built, and what it’s original appearance and total extent was. I think it is the process of trying to answer these questions which may throw up some real and lasting revelations concerning not just Offa’s Dyke itself, but the very origins of Welsh and English culture and society’.” http://www.cpat.org.uk/news/oldnews/offaro.htm