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By Root 2045 0

channel their efforts into those of a respectable trading state. On the continent, the Franks drove the Vikings from the River Seine in A.D. 857, won a major victory at the Battle of Louvain in modern Belgium in 891, and expelled them from Brittany in 939. In the British Isles the Vikings were thrown out of Dublin in 902, and their Danelaw kingdom in England disintegrated in 954, although it was then reconstituted by further raids between 980 and 1016. The year 1066, famous for the Battle of Hastings at which William the Conqueror (William of Normandy) led French-speaking descendants of former Viking raiders to conquer England, can also be taken to mark the end of the Viking raids. The reason why William was able to defeat the English king Harold at Hastings on England’s southeast coast on October 14 was that Harold and his soldiers were exhausted. They had marched 220 miles south in less than three weeks after defeating the last Viking invading army and killing their king at Stamford Bridge in central England on September 25. Thereafter, the Scandinavian kingdoms evolved into normal states trading with other European states and only occasionally indulging in wars, rather than constantly raiding. Medieval Norway became known not for its feared raiders but for its exports of dried codfish.

In light of this history that I have related, how can we explain why the Vikings left their homelands to risk their lives in battle or in such difficult environments as that of Greenland? After millennia of their remaining in Scandinavia and leaving the rest of Europe alone, why did their expansion build up so quickly to a peak after 793, and then grind to a complete halt less than three centuries later? With any historical expansion, one can ask whether it was triggered by “push” (population pressure and lack of opportunities at home), “pull” (good opportunities and empty areas to colonize overseas), or both. Many expansion waves have been driven by a combination of push and pull, and that was also true of the Vikings: they were pushed by population growth and consolidation of royal power at home, and pulled by uninhabited new lands to settle and inhabited but defenseless rich lands to plunder overseas. Similarly, European immigration to North America reached its peak in the 1800s and early 1900s through a combination of push and pull: population growth, famines, and political oppression in Europe pushed immigrants from their homelands, while the availability of almost unlimited fertile farmland and economic opportunities in the United States and Canada pulled them.

As for why the sum of push/pull forces switched so abruptly from unattractive to attractive after A.D. 793, and then subsided so quickly towards 1066, the Viking expansion is a good example of what is termed an autocatalytic process. In chemistry the term catalysis means the speeding-up of a chemical reaction by an added ingredient, such as an enzyme. Some chemical reactions produce a product that also acts as a catalyst, so that the speed of the reaction starts from nothing and then runs away as some product is formed, catalyzing and driving the reaction faster and producing more product which drives the reaction still faster. Such a chain reaction is termed autocatalytic, the prime example being the explosion of an atomic bomb when neutrons in a critical mass of uranium split uranium nuclei to release energy plus more neutrons, which split still more nuclei.

Similarly, in an autocatalytic expansion of a human population, some initial advantages that a people gains (such as technological advantages) bring them profits or discoveries, which in turn stimulate more people to seek profits and discoveries, which result in even more profits and discoveries stimulating even more people to set out, until that people has filled up all the areas available to them with those advantages, at which point the autocatalytic expansion ceases to catalyze itself and runs out of steam. Two specific events set off the Viking chain reaction: the A.D. 793 raid on Lindisfarne Monastery, yielding