Although a bad man, Claudius is a shrewd king and an excellent politician. For, his negotiations with the King of Norway, also the brother of a murdered king--murdered at the hands of King Hamlet, in fact--have gone well. In Act II, Scene 2, the courtiers Voltemand and Cornelius return from Norway and bring news of their success in negotiation. Polonius, Lord Chamberlain, reports to King Claudius,
Th' ambassadors from Norway, my good lord,
Are joyfully returned.(2.2.40-41)
Claudius has successfully avoided invasion by Norway. Voltimand reports that the king's nephew Fortinbras has agreed to not invade Denmark in retaliation for the death of his father and for the loss of certain lands, accepting "threescore thousand crowns in annual fee." Further, the king gives his nephew permission to attack Poland, instead. This Fortinbras agrees to, but asks that he may be "give[n] quiet pass/ Through your [Claudius's] dominions for this enterprise" (2.2.78-79). Claudius tells Voltemand that he will consider this proposal and thanks him.
It seems rather odd that Fortinbras would agree to not attack Norway, then ask permission to cross through it in order to reach Poland. For, if he has completely abandoned hope of avenging his father's death, why would he have any interest in entering the country responsible for this death? Would he not choose an alternate route to Poland? Also, it is curious how much the two countries parallel each other: the kings have been murdered, their brothers then become kings, and the nephews both wish to avenge their father's murder. Clearly, Fortinbras is a foil to Hamlet, and, as such, the reader can expect to encounter him again when he reaches Denmark.
It seem strange that Shakespeare raises the specter of war in Act I and then resolves it so easily in Act II. Here is my suggestion.
Shakespeare had to show the Ghost before the meeting with Hamlet, in order to establish that it was indeed a ghost and that it strongly resembled Hamlet’s father. But Shakespeare wanted the Ghost’s revelations to his son in Act I, Scene 5 to come as a complete surprise to his audience as well as to young Hamlet himself. That was why he brought in the red herring of the troubles with young Fortinbras and the prospect of war with Norway, and why he had the Ghost appear in full armor. (The full armor is clearly intended for deception--to make the audience believe the Ghost is concerned about military matters.) The audience is led to believe that the Ghost’s appearance has something to do with politics and war. (Note that it was Hamlet, Senior who won a duel with the elder Fortinbras and with it some land which young Fortinbras is now seeking to reclaim.) Once Shakespeare had introduced the conflict between Norway and Denmark, he must have decided to develop that subplot plot further, as he continues to do throughout the play.
But the Ghost cares nothing about political or military matters. Why should he? He’s dead! As Macbeth says of another king:
So the audience, like young Hamlet himself, is shocked when they learn the true purpose of Hamlet's dead father's visitation, because they are totally unprepared for what they hear about Claudius’ villainy, having been intentionally misled. Shakespeare knew that if the Ghost began making appearances on the battlements without any apparent purpose, some members of his audience would suspect that he had a serious personal message to deliver to his son, perhaps that he had been murdered. But suddenly, without warning, the audience is informed that Claudius murdered his brother in order to gain the throne and his brother’s wife. They should experience the same kind of shock that Hamlet himself experiences when he hears this news.
Shakespeare had to have some way of making the Ghost look "ghost-like." He did not have the technical resources available in Hollywood. The best he could think of was to dress the actor in a suit armor, so that he would at least look different whenever he appeared onstage. Dressing Hamlet's father in full armor suggested that he was concerned about military matters (a total red herring), which inspired the whole subplot involving hostilities between Denmark and Norway. It seems suggestive that Fortinbras is described as such an enfant terrible but doesn't even appear until the very end of the play.