We should consider that the land-taking, as happened when Hamlet's father conquered Norwegian territory, was quite normal in this day. The fact that Denmark gained land, and that Norway would typically attempt to get it back, was normal and expected. The backdrop of Fortinbras marching towards Denmark to claim what is rightfully his is another thing that causes Hamlet grief as he laments the loss of his father: here's someone, in Fortinbras, who is rightfully avenging his father, while Hamlet struggles with how to do the same thing. Naturally, you could argue that he never really gets to that point, but I think that is best handled in another question or topic.
When Fortinbras arrives, he does indeed realize what all of the back-and-forth land grabbing is good for, as he sees the carnage that has unfolded. Just as Hamlet considered Fortinbras' actions throughout the play, so too do the actions of Hamlet make Fortinbras consider his path. Overall, Fortinbras is yet again shown to be a respectable man, avenging his father as he should, ceasing at nothing to do so (as he should), and then at the end, ultimately changing his mind due to the results of his former course of action.
Fortinbras, the Norweigan prince, who has conquered many other lands, will assume the throne. With him, he also brings the news that Rosencratz and Gildenstern are dead.
In earlier acts he has been shown to be power and land hungry; for example, in 4.4.180-23, the Captain informs Hamlet of the lengths Fortinbras will go to in order to add any acreage at all to the lands he holds. Says the Captain, "We go to gain a little patch of ground / That hath in it no profit but the name. / To pay five ducats, five, I would not farm it; / Nor will it yield to Norway or the Pole. / A ranker rate, should it be sold in fee. "
Then there is the troublesome quarrel between Norway and Denmark, in which Norway intends to regain the lands lost to King Hamlet as the play begins.
But at the end, seeing the carnage vengence and power has wrought, Fortibras sees with new eyes, much like the survivors of Romeo and Juliet learn from their errors. He says, "Take up the bodies. Such a sight as this / Becomes the field but here shows much amiss" (5.2.446-448).
In his recognition, there is some hope for a more peacable time for both Denmark and Norway.