A period of global warming was the impetus for the vikings to venture from their Scandinavian homes. Their boat building skills - the longboat had a low draft, was fast and could be carried over land - and their fearsome reputation as they pillaged and plundered more properous lands meant that around the ninth century they had conquered most of the British Isles, ravaged Europe, made a base in Iceland, colonised Greenland and probably landed in North America centuries before Columbus. They also terrorised Asia Minor and Constantinople.
In Scotland, they had colonised the Shetlands, the Orkneys and the Western Isles and Man. They used as bases to ravage the mainland also colonising the remoter parts like Caithness and Galloway. On these bases they established Jarldoms which in the case of the northern isles lasted until the fifteenth century. It is thought that a prince of the Jarldom of the Western Isles ravaged Dublin and became King Olaf in 853.
It is King Olaf that beseiged Al Cluith in 870, taking it in 871. The Strathclyde king Artgal is taken prisoner and then killed in 872 in a deal done with Constantine of Alba. This was part of Viking policy at the time to hold hostages for ransom, indeed on this occasion 200 longships took booty and prisoners back to Dublin. I.M.M.MacPhail in his Dumbarton Castle explains: "It was in the year 870 that the Norse king of Dublin, Olaf the White, who had established himself there in 853, decided on an expedition to plunder the kingdom of the Britons in Strathclyde. He set off with a large fleet from Dublin and, sailing up the Firth of Clyde, laid seige to Alclut. He was joined by another Viking ruler, Ivar Beinlaus ('cripple' or 'one-legged'), who came north from York, which he had seized in 867. The garrison of Alclut held out for four months but at length was compelled to surrender, as the well on the rock had dried up ('miraculously', according to one annal, or by the Norsemen 'wonderfully' drawing off the water, according to another). The citadel was destroyed and the kingdom of the Britons lay prostrate before the invaders, who remained in Strathclyde over the winter, sailing back to Dublin with a fleet of two hundred ships laden with slaves and booty. The king of Strathclyde was killed shortly afterwards and the kingdom for a time passed under the control of neighbouring kings." An excavation in 1975 discovered a vitrified rampart that Professor Alcock has suggested dated from this viking seige. MacPhail further notes:".. in the debris were found a sword pommel of a type similar to that used in western Ireland at that period and fragments of a glass bangle set in a lead matrix, similar to a type used by the Vikings."
This marks the rise of Govan (and hence Glasgow) as the main administrative centre of Strathclyde. Furthermore, the similarity of the layout of Govan and that of the Tynwald on the Isle of Man suggest Norse influence. Some of the tombs in the churchyard can also be associated with the Scandinavians. It seems likely that the British royal family married with the Scandinavians in recognition of their power base.
The last time the vikings were to trouble this area was previous to the Battle of Largs in 1263. It is doubtful however if they managed to penetrate as far south as Drumchapel. More on this can be read in entry of King Alexander III.
Although the vikings maintained various Jarldoms, at the distances involved they could not maintain a administrative base for such a large empire. Although they had a beneficial effect in increasing trade and developing towns and cities, ultimately each Jarldom would be absorbed back by its larger neighbour; hence Scotland eventually recovered the Orkneys, Shetland, the Western Isles and Man. By the time of the start of the fourteenth century and the start of the little ice age, which closed off many viking sea routes, the viking's zenith was already a distant memory; the victory of the Battle of Largs in 1263 ceded the Western Isles to Scotland, the Treaty of Perth gave Scotland Man in 1266 although it took a battle in 1275 to confirm its status, and in 1283 the marriage of King Alexander III's daughter to King Eric of Norway had secured peace. By another marriage, that of King James III to the daughter of the King of Norway in 1469, the Orkneys and Shetland became formally part of Scotland.