] As I mentioned last week, I find it very interesting that Fortinbras is the person left standing at the end of the play.

Well, he isn't. Horatio is standing too, but that's another thread! :-)

] Is the message supposed to be that, to be a successful ruler, you had to be annexing new and regaining formerly lost territory all the time, as King Hamlet and Fortinbras have done?

To some extent, I would say, yes. But I think F is there for more than a political statement by Shakespeare. He is there as a catalyst for H. F also gives H some inspiration when he finds F's armies ready to cross over Denmark and invade Poland.

How all occasions do inform against me,
And spur my dull revenge!

If Hamlet had intellectual reason not to act, and he has spent three hours finding reasons not to actually act against Claudius, he is resolved after this point to do so. F also indirectly shows H what it means to be a "great" leader.

Rightly to be great
Is not to stir without great argument,
But greatly to find quarrel in a straw
When honour's at the stake.

As far as I see it, this means that H realizes that to be great is not acting without thinking, but acting decisively on intensely thought out and isolated matters of honor. I think this is a turning point for H brought about by F.

So, although I think Shakespeare may be making a larger statement, I think F is there to be a catalyst for H.