The backdrop of war provides a tense undercurrent to Hamlet. The reason that Denmark is preparing for war is because Norway is preparing to invade Denmark. Horatio explains this in the first scene of the play. He says that King Hamlet had killed Fortinbras (the King of Norway) in one-on-combat and taken territory from him. Under feudal laws, the territory rightfully belonged to King Hamlet, but the son of Fortinbras, known as Young Fortinbras, was full of "unimproved metal hot and full" and was assembling an army of "lawless resolutes" to avenge his father's death and take back the lands he lost.

Complicating matters was the fact that Young Fortinbras's uncle had assumed the throne of Norway, a situation that parallels Hamlet's own predicament in Denmark. Indeed, the decisive Young Fortinbras serves as sort of dramatic contrast to Hamlet. Hamlet who vacillates and delays his quest to avenge his own father's death. Horatio and the other men on the watch attribute the appearance of King Hamlet's ghost to the crisis that confronts Denmark. Claudius sends ministers to the King of Norway, who reins in his hot-headed and ambitious nephew. Young Fortinbras then determines to invade Poland by way of Denmark.

At the end of the play, this subplot is resolved as Young Fortinbras, while passing through Denmark, encounters the aftermath of Hamlet's duel with Laertes. Hamlet, as he lays dying, gives the young Norwegian his blessing to become King of Denmark.