In hamlet, why are there preparations for war?

Hamlet's father won certain lands that belonged to Norway, so after Hamlet's father dies and his uncle Claudius takes over, Fortinbras (Prince of Norway) wants to reconquer those forfeited lands. Fortinbras goes ahead with an army and we learn this when Hamlet comes across a captain at one point and he informs Hamlet that the Norwegian army is planning an attack. Hamlet asks about the basis of the conflict and the man tells him that the armies will fight over “a little patch of land / That hath in it no profit but the name” (IV.iv.98–99). The Norwegian King gets mad at Fortinbras for attempting war on Denmark so Fortinbras said he'd never do it again. The King of Norway tells him to use his army to attack the Poles instead of Denmark, so he does that.