I was thinking about viking raids, their later invasions of bigger areas, pitched battles against Anglo-Saxons, Franks, their civil wars... And i thought I might ask you people what do you think were viking greatest military achievements? Conquest of most of the Anglo Saxon kingdoms in the second half of the 9th century? Bullying Franks into giving them Normandy? If we count that as viking invasion, then maybe conquest of England by Svein and Cnut in 11th century? Maybe Harald the Fairhair's wars against petty kings and various jarls to unify Norway? Maybe viking conquests in the East?
What do you think? I guess there are some battles or conquests I left out here...

I read a bit more about Svein's and Cnut's invasion of England and it seems that much of England submitted to them without much fighting, there was only one great battle at Assandun and rather indecisive fighting for London. Do you think England might have resisted if more unified? Why were they so afraid of the Danes? Was it the lack of determined leadership?

I think that the lack of leadership certainly had a good bit to do with it.
But also look at the way the Vikings were fighting: it was guerrilla warfare in large numbers. If an area looked too heavily defended, well, keep on sailing on to the next 'soft' town. Come back to the first town once all their neighbors have been killed, captured, surrendered, or fled.
It made local support iffy. "We can't leave our town to help so and so because they (vikings) may come here next to attack our undefended homes."
This is the attitude I get from reading anyway, to be taken with a grain of salt twice daily.

I think the repeated defections of Eadric were a majour factor why England fell to Canute. Had he not changed sides at Assandun, the conflict might have ended differently.