In the opening scene of the play Horatio explains the present crisis to Marcellus and Bernardo in detail. Horatio is well educated and better informed than these common soldiers who are only following orders from their superiors and can't see the big picture. According to Horatio, Denmark is being invaded by a substantial Norwegian army and there is danger of a full-scale war.
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 return'd
To the inheritance of Fortinbras,
Had he been vanquisher; as, by the same covenant,
And carriage of the article design'd,
His fell to Hamlet. Now, sir, young Fortinbras,
Of unimproved mettle hot and full,
Hath in the skirts of Norway here and there
Shark'd 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.
It would appear, however, that Shakespeare is using this military subplot as a distraction to keep the audience from guessing why a spectral figure who appears to be the ghost of Hamlet's dead father is appearing on the battlements dressed in his old armor. Shakespeare apparently intended to surprise both Hamlet and the audience with the revelation of the true purpose of the ghost's visitations. The Ghost is not concerned about the possible war. He is wearing armor only because that was how Shakespeare was trying to make him look "different," "ghost-like." Otherwise, the audience would just think he was a live actor playing a living character. One of Shakespeare's major problems in this scene is to establish this man in armor as a ghost and evidently the ghost of the recently deceased King Hamlet, young Hamlet's father, who was supposedly killed by a poisonous snake while asleep in his garden. Naturally the men on guard duty are wondering what the ghost wants and why he is appearing at this specific time. The audience would, of course, be wondering too. They are misled to believe that the ghost is concerned about a possible invasion--but the old King Hamlet is dead and beyond the pale. He is not concerned about such military or political matters anymore. The dead are probably relieved to leave such problems to the living, because there will always be problems of that genre to contend with. No doubt the actor was instructed to behave in a "spooky" manner, stalking slowly and silently, staring straight ahead. He was probably wearing only stockings or felt slippers so that his footsteps would be silent on the telltale wooden boards.
In Scene 5 of the first act, the Ghost tells his son Hamlet the real purpose of his visit. He reveals that he was murdered by his brother Claudius and wants Hamlet to kill Claudius in revenge. This news is astonishing to Hamlet and everyone in the house because they thought the Ghost was there because of the threat of war with the Norwegians led by the ambitious young Fortinbras.
Once Shakespeare introduced the subplot of the quarrel with Fortinbras, he must have decided to make further use of it. But it is of little importance in comparison to the main plot, which involves Hamlet and Claudius playing cat-and-mouse while Hamlet is driven by his sworn duty to murder the King. The fact that the threatening war with the Norwegians was essentially a "red herring" is proven in the second scene of the second act. The whole issue has been resolved without any military confrontation. Fortinbras' uncle has ordered him to cease and desist his provocations. Voltimand and Cornelius, Danish ambassadors, return from Norway with news that the matter is settled. Voltimand reports:
Most fair return of greetings and desires.
Upon our first, he sent out to suppress
His nephew's levies; which to him appear'd
To be a preparation 'gainst the Polack;
But, better look'd into, he truly found
It was against your highness: whereat grieved,
That so his sickness, age and impotence
Was falsely borne in hand, sends out arrests
On Fortinbras; which he, in brief, obeys;
Receives rebuke from Norway, and in fine
Makes vow before his uncle never more
To give the assay of arms against your majesty.
Whereon old Norway, overcome with joy,
Gives him three thousand crowns in annual fee,
And his commission to employ those soldiers,
So levied as before, against the Polack:
With an entreaty, herein further shown...