Wouldn’t state it as a sign of military art. King Charles The Bald had dificult reign, as he had problems with internal disputes, disputes with his brothers and even more, Vikings raiding his realm. He wasn’t able to assemble large enough army to effectively use defense network built by Charlemagne and deny Vikings to attack core of Frankia. Several sackings of Paris during 840s and 860s were results of false planning and lack of Frankia to unite and hold incoming outer enemy imho. Charlemagne was able to unite Frankish tribes and form them into state, that means he had working army that could protect his empire from northern threat. His successors actually failed in this and first who succesfully defended Paris was Odo during 885-6 siege.
Again, same as in Frankia. No real big threat, no strong leader that could unite whole Anglia to actualy stop invaders. British Isles were full of petty kingdoms and duchies, remnants of fallen Roman Empire.
This is what was used during whole middle ages. Most of warfare took place during summer, because it was much easier to obtain supplies to keep marching that colossus of several (tens of) thousands soldiers. And I believe that meeting of knights of high middle ages era with vikings of their era, would end conclusively in knights success. But this is what I think is uncomparable, because right now we are comparing two “doctrinas” that are divided by 200-400 years of military advancements.
Back to the topic.
Basically fully equipped knight is heavy infantry or heavy cavalry, while vikings were light infantry. Mobile and deadly, but single blow from a knight makes end to him. And those people, that say “faster swordsman is better than armored one” can go to (insert some shiny place with flowers and birds here). Heavily equipped knight losses his breath after longer fight, not instantly after first two blows (he trains that whole life!), light foot can withstand this longer, but usually he gets hit much sooner. That would have to be fight of heavily equipped greenie against lightly equipped vet. Something like me standing in full armor against Anton Kohutovič => jelly good show! Lasting few seconds…
And from what I saw, Samurais were in fact light infantry/cav, compared to mid European knights…