What current events are discussed as the men await the ghost in Act I Scene i?

Horatio, Marcellus, and Bernardo are waiting for the ghost of the last king of Denmark, Hamlet the elder, to appear.  Bernardo and Marcellus had seen the ghost before, and brought Horatio, the scholar, to see it because Horatio had not believed them.  When Horatio is confronted with the very image of the dead king walking toward him, he fully believes:

Before my God, I might not this believe
Without the sensible and true avouch
Of mine own eyes.(70) (I.i)

Then the three men discuss how very like the ghost looked like Old Hamlet "When he the ambitious Norway combated" (74) in the last war with that country.  Also they mention that the ghost frowned exactly like how the king had when he had beaten the Polish army upon the ice.  Horatio immediately jumps to the conclusion that the appearance of the old king's ghost in "fair and warlike form" (58) must mean "This bodes some strange eruption to our state".  Horatio believes that this must mean bad things for the military security of Denmark.

Horatio, Marcellus, and Bernardo continue their discussion of how the country is arming and preparing for war.  Horatio gives a long speech, the upshot of which is that Norway is currently arming and maintaining a mercenary army to take back, by conquest, a quantity of land which Old Hamlet had taken in war from them during his lifetime.   The terms of the treaty had been non-negotiable, so, in order to get it back, young Fortinbras (the nephew of the old king of Norway who had made the agreement) has no other option than to take it by force.  That is why there is a watch and such commotion of war-mongering right now.  

Bernardo and Marcellus are inclined to agree with Horatio.  Horatio reminds them that there were supernatural signs and portents in the city of ancient Rome before Julius Caesar died; therefore, this ghostly visitation must mean something dire for the kingdom.  When the ghost returns, Horatio asks it directly if it knows the future of Denmark:

Speak to me;
If thou art privy to thy country's fate,
Which, happily, foreknowing may avoid,
O, speak! (146-49)

The cock crows, heralding dawn, and the ghost disappears.  The ghost has told them nothing, but Horatio and the others believe its appearance was a sign for the future of the country, rather than the restlessness of a unavenged murdered man.  The men depart, and resolve to tell young Hamlet, for they believe that the ghost will speak to his own son.