I could not help but notice that the only one who survives to tell the tale is Horatio, so we really only have HIS version of what happened. Which is ludicrous in the extreme. So here is what I think went down:
Fortinbras wants to invade Denmark to recapture the lands lost by his father. He gathers an army and is ready to march when the King of Norway stops him, at Denmark’s request. So Fortinbras makes up the ridiculous excuse that he just wants to march through Denmark enroute to Poland, though it is obvious to everyone that the targeted Polish territory is worthless--so clearly just an excuse to enter Denmark with his 20,000 troops. Then Fortinbras sends his agent Horatio to Denmark where Horatio murders the entire royal family and the chief minister and his daughter and son (the only witnesses) and invents this completely ridiculous story of how he--Horatio, friend to Hamlet--helped uncover the murder of Hamlet’s father by Hamlet’s uncle and the uncle’s marriage to the queen (Horatio thus undermining the legitimacy of the then current rulers) and how the current king’s attempt to murder Hamlet led to the deaths of the rest of the royal family and advisors. Thus, when Fortinbras arrives (conveniently timed to the exact moment to find the entire royal family dead of poison, the chief minister stabbed, and his daughter drowned) Fortinbras is well positioned to take over Denmark. With all the legitimate claimants to the throne dead simultaneously, and backed by a mobilized army of 20,000 already in place in Denmark with the Danes disorganized, rulerless, and the royal line discredited, Fortinbras is the only winner in this scenario…
What other possible explanation for events is there? Because clearly, Horatio’s versions of the events is so over-complicated as to be completely unbelievable.