what military concerns do they discuss?

In Act I, Scene 1, Marcellus asks Bernardo and Horatio why there has been such a massive mobilization of Danish war forces recently. Horatio answers, saying that the Danish army is preparing for a possible invasion by Fortinbras, Prince of Norway. We learn that Fortinbras’ father (also named Fortinbras), was killed many years before in single combat with Old Hamlet, the now-deceased king whose ghost we have just seen. Now that Old Hamlet has died, presumably weakening the Danes, there is a rumor that Fortinbras plans to invade Denmark and claim that lands that were forfeit after his father’s death.