In Hamlet, Norway and Denmark are heading towards war.
As explained in the play by Horatio, King Hamlet (not Prince Hamlet, but his father) of Denmark slew King Fortinbras of Norway and captured land (the two countries have a complex and close relationship in real history too, variously competing over land and titles, joining in union, or one holding dominion over the other - normally Denmark being the 'bigger brother').
During the events of Hamlet, although King Fortinbras' brother is on the throne, he is bedridden and instead King Fortinbras' son (also called Fortinbras - it gets confusing!) seeks revenge. The younger Fortinbras is massing an army to attack Denmark, reclaim the disputed land, and get venegance for his father's death at the hands of King Hamlet (senior). He does not have official permission from the current king of Norway for this - he's going rogue, essentially (in keeping with the play's theme of revenge).
Hope this is a helpful summary, and puts the Hamlet Jr / Fortinbras Jr relationship - and its very different resolution to that of the their fathers - in a little more context!