The business of our Vikings was coastal sting operations, and they excelled at it.
Being itinerant tribal warriors from the far fringe of Europe, they didn’t get the occasion to form an orderly military structure that could deploy forces for a major battle, like the mighty Charlemagne or the Arab conquerors did. They avoided major battle on our territory so that no one had a chance to defeat them and chase out.
First when they settled in Northern France, in Kievan Rus and Sicily and blended with local elites, they started to think and act like regular kings and dukes and counts. And also at this, they were hugely successful, with at least three mighty kingdoms as their lasting legacy.
The disappearance of Vikings outside Scandinavia wasn’t due to their defeat. What happened?
The fierce Nordic warriors had strong preference for indigenous ladies. They had kids with them and found the entire bunch so irresistibly sweet that they didn’t mind them talking to each other in their MOTHER tongue.
In a couple of generations, assimilation left few traces of the Nordic legacy, wherever they had settled.
Love conquers all, folks!