The Ghost makes its first appearance before the Marcellus, Bernardo and Horatio have had much conversation. After it disappears, the only subject involving current events has to do with the possibility of war. The most pertinent dialogue is as follows:
Good now, sit down, and tell me, he that knows,
Why this same strict and most observant watch
So nightly toils the subject of the land,
And why such daily cast of brazen cannon,
And foreign mart for implements of war,
Why such impress of shipwrights, whose sore task
Does not divide the Sunday from the week.
What might be toward, that this sweaty haste
Doth make the night joint-labourer with the day?
Who is't that can inform me?
That can I;
At least the whisper goes so. Our last King,
Whose image even but now appear'd to us,
Was, as you know, by Fortinbras of Norway,
Thereto prick'd on by a most emulate pride,
Dared to the combat; in which our valiant Hamlet--
For so this side of our known world esteem'd him--
Did slay this Fortinbras; who by a seal'd compact,
Well ratified by law and heraldry,
Did forfeit, with his life, all those his lands
Which he stood seized of, to the conqueror;
Against the which, a moiety competent
Was gaged by our King; which had returned
To the inheritance of Fortinbras,
Had he been vanquisher--as, by the same cov'nant
And carriage of the article designed
His fell to Hamlet. Now, sir, 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--
As it doth well appear unto our state--
But to recover of us, by strong hand
And terms compulsatory, those foresaid lands
So by his father lost. And this, I take it,
Is the main motive of our preparations,
The source of this our watch and the chief head
Of this post-haste and romage in the land.
Briefly, young Fortinbras, heir apparent to the throne of Norway, is raising an army to reclaim some land that his father lost to the dead King Hamlet in a duel. The three men discussing these matters believe that the appearance of King Hamlet's ghost must have something to do with the pending military conflict. However, it turns out that the Ghost is there for an entirely different reason. He wants to talk to his son and tell him, as he will do in Act 1, Scene 5, that he was murdered by his brother Claudius who is now the king of Denmark and that his son is bound to exact revenge.
The introduction of the problem with Fortinbras is evidently intended to mislead the audience. Shakespeare apparently wanted the news of King Hamlet's murder to come as a complete surprise, so he invented a sort of subplot which turns out to be a false alarm. The whole matter involving Fortinbras is settled without bloodshed. Fortinbras is told by his uncle the king of Norway to cease and desist, and he immediately obeys. However, he receives permission to remain in Denmark to conduct another military adventure in Poland.
The Ghost appears again right after Horatio has explained what he believes to be the reason for its visitation, and this reinforces the impression that the Ghost, especially since it is wearing armor and a helmet, is concerned about military matters and not personal ones. Shakespeare did not want anyone in the audience, or even any of the three men on guard duty, to suspect that the Ghost might have something of importance to communicate to his son Hamlet. Marcellus, Bernardo, and Horatio only tell Hamlet about the incident because they believe the Ghost will speak to his son even though it will not speak to any of them. After Hamlet has talked to his dead father at length in Act 1, Scene 5, he swears Marcellus, Bernardo and Horatio to secrecy, because it is of the utmost importance that King Claudius, the assassin and usurper, should not get any hint that Hamlet has been in communication with the ghost of his father.
The subjects that are being discussed is the chance of possible war from Fortinbras. When they are talking about this they see the ghost of Hamlet's father. Hamlet is still grieving over the fact his father died and because of this is very unhappy. When they see his father, they tell Hamlet what they saw and Hamlet is surprised and wants to see this for himself.
the subjects that are being discussed have to do something with the possibliyof war.