Hamlet begins with two guards and Horatio discussing recent events in Denmark, the most notable being the ongoing conflict with Fortinbras of Norway. Horatio explains the backstory to the guards in Act I, Scene i, lines 79–107. The main points of Horatio’s speech explains that Hamlet Senior, the titular Hamlet’s dad, was challenged to combat by Fortinbras of Norway and won. Because Hamlet Senior beat Fortinbras, according to Fortinbras and Hamlet Senior’s agreement, Fortinbras’s land goes to Hamlet Senior. Now Fortinbras’s son, young Fortinbras, wants to avenge his father and get back his family’s land. Horatio notes that no one really knows yet how good at fighting Fortinbras Junior is, but he’s gotten together a gang of men in Norway and is now stirring up trouble. This backstory, as Horatio notes at the very end of this speech, explains why Marcellus, Barnardo, and Horatio are even keeping watch in the first place. An outside force is threatening Denmark and the warlike king, Hamlet Senior, is no longer there to protect the kingdom. People are nervous. Additionally the threat of upsetting the current kingdom comes from a son, upset by his father’s death and the loss of his birthright, a theme we see again throughout the play with Hamlet and Laertes.
The ghost’s appearance supports this because he is dressed for battle. Horatio tells Hamlet in Act I, Scene ii, line 200, the ghost is “Armed at point.” This must be important because they return to this detail later, in this exchange from lines 226–230:
Hamlet Armed you say?
All Armed, my lord.
Hamlet From top to toe?
All My lord, from head to foot.
Hamlet Then saw you not his face.
Horatio O, yes, my lord. He wore his beaver up.
Hamlet Senior appears not just as a ghost, but as a ghost prepared for battle, wearing armor from head to foot and with his visor up. His warlike outfit suggests that he is anticipating a conflict and has come to wage war, both against Fortinbras Junior and against Claudius.
This is the point that Horatio raises in his allusion to Julius Caesar in Act I, Scene i, lines 112–125. He reminds Marcellus and Barnardo that right before Julius’s reign was violently ended there were strange, supernatural occurrences, like empty graves, the dead reappearing in the streets, shooting stars, dews of blood, disorder in the movements of the sun and moon. He notes that these weird occurrences seemed to predict that something bad was about to happen, and says that Hamlet Senior’s armored appearance is “prologue to the omen coming on.”