The Norwegians are attempting to invade Denmark; Hamlet killed their king
Explanation:
The Norwegians attack Denmark during the duration of the play because the King Hamlet, who is the father of Prince Hamlet killed the king of Norway in a battle for winning the lost territory from them.
Since then the crown prince, Fortinbras had been biding time to fight back and take over Denmark an opportunity to which he gets from the strife between Claudius who is a weak king and hamlet who as a prince is very indecisive and his mind is diverted by his own troubles.
