well-known surprise attacks ?
I can't make up the sense of "well-known surprise attacks :)

Context:

Historically, Sweden has an interesting story. Its dealings with the outside world began, in fact, during Viking times, when in addition to the well- known surprise attacks of the nearby lands, there was much trading around the Baltic, mostly in furs and weapons. Swedish connections with the other Scandinavian countries, Norway and Denmark, have been strong since the Middle Ages. The monarchies of all three are still closely linked.

Thanks a lot.

Question

well-known surprise attacks ?
I can't make up the sense of "well-known surprise attacks :)

Context:

Historically, Sweden has an interesting story. Its dealings with the outside world began, in fact, during Viking times, when in addition to the well- known surprise attacks of the nearby lands, there was much trading around the Baltic, mostly in furs and weapons. Swedish connections with the other Scandinavian countries, Norway and Denmark, have been strong since the Middle Ages. The monarchies of all three are still closely linked.

Thanks a lot.

Answer

It is an interesting phrase.  I'll make up an illustration for you.  If you had a little boy living on your street,  and  he liked to jump out from behind a car and throw snowballs at you in the winter.  If he did this a lot (every day,  or 3-4 times a week),  you would come to expect it.  You may not know exactly when he might attack you with snowballs,  but you know it's going to happen sooner or later.  You could describe them as well-known surprise attacks.

In the context you supplied,  the vikings were well known for their raids on different places.  These raids were expected, but no one knew exactly when  or where they would take place.  They were well-known surprise attacks.

Answer

It is well-known that the Vikings conducted surprise attacks on nearby lands.