second case is the internal "Push" factor, which coincides with a period just before the Viking Age in which Scandinavia was undergoing a mass centralization of power in the modern-day countries of Denmark, Sweden, and especially Norway. This centralization of power forced hundreds of chieftains from their lands, which were slowly being eaten up by the kings and dynasties that began to emerge. As a result, many of these chiefs sought refuge elsewhere, and began harrying the coasts of the British Isles and Western Europe. Anders Winroth argues that purposeful choices by warlords "propelled the Viking Age movement of people from Scandinavia." Technological model This model suggests that the Viking Age occurred as a result of technological innovations that allowed the Vikings to go on their raids in the first place. There is no doubt that piracy existed in the Baltic before the Viking Age, but developments in sailing technology and practice made it possible for early Viking raiders to attack lands farther away. Among these developments are included the use of larger sails, tacking practices, and 24-hour sailing. Anders Winroth writes, "If early medieval Scandinavians had not become exquisite shipwrights, there would have been no Vikings and no Viking Age." These models constitute much of what is known about the motivations for and the causes of the Viking Age. In all likelihood, the beginning of this age was the result of some combination of the aforementioned models. The Viking colonization of islands in the North Atlantic has in part been attributed to a period of favorable climate (the Medieval Climactic Optimum), as the weather was relatively stable and predictable, with calm seas. Sea ice was rare, harvests were typically strong, and fishing conditions were good. Historic overview The earliest date given for a Viking raid is 789, when according to the Anglo-Saxon Chronicle, a group of Danes sailed to the Isle of Portland in Dorset (it was wrongly recorded as 787). They were mistaken for merchants by a royal official. When asked to come to the king's manor to pay a trading tax on their goods, they murdered the official. The beginning of the Viking Age in the British Isles is often set at 793. It was recorded in the Anglo–Saxon Chronicle that the Northmen raided the important island monastery of Lindisfarne (the generally accepted date is actually 8 June, not January): In , according to the Annals of