From Empires of the Atlantic World: Britain and Spain in America 1492-1830, by John H. Elliott (Yale U. Press, 2006), Kindle Loc. 437-457:
Medieval England pursued a policy of aggressive expansion into the non-English areas of the British Isles, warring with its Welsh, Scottish and Irish neighbours and establishing communities of English settlers who would advance English interests and promote English values on alien Celtic soil. The English, therefore, were no strangers to colonization, combining it with attempts at conquest which brought mixed results. Failure against Scotland was balanced by eventual success in Wales, which was formally incorporated in 1536 into the Crown of England, itself now held by a Welsh dynasty. Across the sea the English struggled over the centuries with only limited success to subjugate Gaelic Ireland and `plant’ it with settlers from England. Many of the lands seized by the Normans in the twelfth and thirteenth centuries were recovered by the Irish during the fourteenth and fifteenth; and although in 1540 Henry VIII elevated Ireland to the status of a kingdom, English authority remained precarious or non-existent beyond the densely populated and rich agricultural area of the Pale. With the conversion of Henry’s England to Protestantism the effective assertion of this authority over a resolutely Catholic Ireland acquired a new urgency in English eyes. The reign of Elizabeth was to see an intensified planting of new colonies on Irish soil, and, in due course, a new war of conquest. The process of the settlement and subjugation of Ireland by the England of Elizabeth, pursued over several decades, absorbed national energies and resources that might otherwise have been directed more intensively, and at an earlier stage, to the founding of settlements on the other side of the Atlantic.
In medieval Spain, the land of the Reconquista, the pattern of combined conquest and colonization was equally well established. The Reconquista was a prolonged struggle over many centuries to free the soil of the Iberian peninsula from Moorish domination. At once a military and a religious enterprise, it was a war for booty, land and vassals, and a crusade to recover for the Christians the vast areas of territory that had been lost to Islam. But it also involved a massive migration of people, as the crown allocated large tracts of land to individual nobles, to the military-religious orders engaged in the process of reconquest, and to city councils, which were given jurisdiction over large hinterlands. Attracted by the new opportunities, artisans and peasants moved southwards in large numbers from northern and central Castile to fill the empty spaces. In Spain, as in the British Isles, the process of conquest and settlement helped to establish forms of behaviour, and create habits of mind, easily transportable to distant parts of the world in the dawning age of European overseas expansion.
The conquest and settlement of Al-Andalus and Ireland were still far from complete when fourteenth-century Europeans embarked on the exploration of the hitherto unexplored waters and islands of the African and eastern Atlantic. Here the Portuguese were the pioneers. It was the combined desire of Portuguese merchants for new markets and of nobles for new estates and vassals that provided the impetus for the first sustained drive for overseas empire in the history of Early Modern Europe. Where the Portuguese pointed the way, others followed. The kings of Castile, in particular, could not afford to let their Portuguese cousins steal a march on them. The Castilian conquest and occupation of the Canary Islands between 1478 and 1493 constituted a direct response by the Crown of Castile to the challenge posed by the spectacular expansion of Portuguese power and wealth.