I'm watching the TV Drama Vikings, and I know much of it is unrealistic however entertaining, but how does one become a noble in that time? Continental and the British Island were much easier to understand, but the show depicted as if you just need to kill the noble, which seemed weird.
While there will always more to be said, I hope that my following previous posts might offer some basic ideas on the rulership and military commands in Viking Age Scandinavia:
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How to translate different ruler titles and their exact concepts in Viking Age Scandinavia either into modern English or in contemporary Latin has often been not so easy.
Put it simply, in contrast to famous representation of the kingdoms of the Vikings either in popular medias, such as TV series Vikings and "historical" games like Crusader Kings, there had been not any stable, politically unified Scandinavian (Nordic) kingdom with dynastic continuity until the late Viking Ages (late 10th century), possibly except for the land of the Danes under the reign of Gudfred "the Great" (d. 810) in the early 9th century. To give an example, in spite of his Old Norse konungr style, scholars think that konungr Sygtryggr (early to middle of 10th century?), mentioned in the first of three linked post above, controlled at most southern or south-eastern part of Jutland Peninsula only. Near contemporary texts like Adam of Bremen (written about 1075) mention several ruler names even only within Jutland Peninsula, but modern scholars often find it difficult to identify them with other fragmentary evidence like runic inscriptions. Instead of a single kingdom, it would be better to think that there were many petty-kingdoms and sometimes their alliance that dominated the political landscape of Viking Age Scandinavia.
Though not all, but a few researchers also argue that this unstable power politics of the Scandinavians can explain at least part of the apparent failure of the 9th century Frankish diplomacy against the waves of the Vikings - Just to negotiate with the single ruler "of the Danes" in Denmark was not suffice to stop the invading fleet, since the ally was not responsible for the attacks planned by his rival (petty-) kings or independent local magnates!
As I explained before in the second linked post, the basic principle of military recruitment of the expedition in Viking Age Scandinavia was the military retinue (called lið in Old Norse) of individual ruler's household (hirð). Some powerful rulers, however, were known to call their allied rulers either for the defense or for the expedition. Thus, larger scale of their fleet, such as the Great Army ravaging in the late 9th century England is now considered essentially as a assemblage of the military retinue of the chieftains that joined in the expedition, either willingly or not so willingly, but on the request from his partner. Even in the 11th century, the main "political power" exerted by the really powerful "Viking King" in Scandinavia like King Sweyn Forkbeard of the Danes (d. 1014), sometimes called "overlordship" by researchers, in fact rely on the aggregation of such personal alliances.
In such politico-social circumstances, to kill the "king", or the key figure in the complicated alliance networks among the petty-rulers would probably lead to their dissolution, represented by the aggregated fleet of the Vikings. Another ruler who had alternative accumulation of alliances would probably took over his place, but it was not necessarily his son or dynastic members (though they might have had some advantages due to the remaining alliances).
It was at least not until the late 11th century or Norway and Denmark to wait for more formal form of succession - even at that point, we should perhaps not overestimate the institutional aspect of their politics (Hermanson 2000). In Sweden, succession strife and political turbulence would probably continue well into the most period of the 12th and later......
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