The "Norwegian situation" as you put it, is a danger that is facing Denmark at this point in the play.
At this point, young Fortinbras is the nephew of the king of Norway. The king is sick and does not (Claudius says) really know what his nephew is doing. What Fortinbras is doing is getting troops together to avenge the fact that the previous king of Denmark (Hamlet, father of the Hamlet in the play) defeated Fortinbras's father in battle. He also wants to take back land that the late King Hamlet took.
Claudius is writing to the Norwegian king asking him to keep his nephew under control.