explanation for Somairle's invasion is that he may have been compelled to counter a threat that Walter—and other recently-enfeoffed Scottish magnates—posed to his authority. A catalyst of this collision of competing spheres of influence may have been the vacuum left by Óláfr's assassination. Although the political uncertainty following Óláfr's elimination would have certainly posed a threat to the Scots, the concurrent build-up of Scottish power along the western seaboard—particularly exemplified by Walter's expansive territorial grants in the region—meant that the Scots were also positioned to capitalise upon the situation. In fact, there is reason to suspect that, during Malcolm's reign—and perhaps with Malcolm's consent—Walter began to extend his own authority into the Firth of Clyde, the islands of the Clyde, the southern shores of Cowal, and the fringes of Argyll. The allotment of Scottish fiefs along the western seaboard suggests that these lands were settled in the context of defending the Scottish realm from external threats in Galloway and the Isles. It was probably in this context that substantial western lordships were granted to men such as Hugh de Morville, Robert de Brus, and Walter himself. As such, the mid-part of the twelfth century saw a steady consolidation of Scottish power along the western seaboard by some of the realm's greatest magnates—men who could well have encroached into Somairle's sphere of influence. The continuous encroachment of Scottish authority may well have spurned Somairle to launch a counter-strike. It is conceivable that Somairle first acquired the islands of the Firth of Clyde after his 1156 clash with Guðrøðr. In so doing, Somairle gained control of islands in a territory that appears to have been regarded by the Scots as vital to their own security. In fact, the catalyst for the establishment of Scottish castles along the River Clyde could well have been the potential threat posed by Somairle. Somairle's final campaign appears similar to later Norwegian-backed invasions of the Firth of Clyde conducted by his descendants in the thirteenth century (1230 and 1263). In fact, the Viking sack of Alt Clut in 870 may parallel these invasions, since it is possible that Dublin-based Vikings destroyed the fortress of Alt Clut (Dumbarton Castle) in an effort to nullify a threat posed by the Strathclyde Britons. Another factor that may have spurned Somairle to attack the Scots could have been Malcolm's increasingly poor health. Certainly, the Chronicle of Melrose states that the Scottish