In act 1, scene 1, Barnardo and Marcellus have recruited Hamlet's friend Horatio to come up to the castle ramparts and wait for the ghost of the old King Hamlet (who is dressed in full armor, as though ready for battle). Horatio, even before he sees the ghost, thinks that the ghost's appearance may be related to the threat of war from Norway, in part because of the preparations that Denmark appears to be making for such a war. He explains that, in the recent past, the king of Norway, old Fortinbras, started a war with old King Hamlet, and the Danish forces defeated the Norwegian armies; during this time,

[Old Hamlet] Did slay this Fortinbras, who by a sealed compact,
Well ratified by law and heraldry,
Did forfeit, with his life, all those his lands
which he stood seized of, to the conquerer (1.1.98–101).

In other words, Norway lost not only its king, it also lost a great deal of land to Denmark as a result of the conflict. Now, according to Horatio,

Young Fortinbras,
Of unimproved mettle hot and full,
Hath in the skirts of Norway here and there
Sharked up a list of lawless resolutes
For food and diet to some enterprise
That hath a stomach in 't; which is no other . . .
But to recover of us, by strong hand
And terms compulsatory, those foresaid lands
So by his father lost (1.1.107–116).

The dead king's son, young Fortinbras, appears to be attempting to avenge his father's death and the losses his country sustained as a result of this defeat. He is rounding up whoever is willing to fight with him to attack Denmark and to try to win back the lands that Norway lost.