Conquered by Alexander the Great, the Kingdom of Darius did not rebel against his successors after his death. Why not?
Now you're thinking: didn't Alexander the Great get land quickly and hold on to it even after his death? And weren't you just saying how difficult it is to hold on to new lands, Machiavelli?
Well, there's a perfectly reasonable explanation for that.
There are two ways to govern a monarchy: by a king and non-elected barons or by a king and his appointed ministers. (Barons are big-shot nobles or ministers, like the president's Cabinet, only hereditary and with their own land and armies.)
In the second one, the king is more powerful since he's the only person who has real power.
Turkey is one example of this kind of government. Because the king has all the power, a country like Turkey is pretty hard to conquer. There are no barons to help you by turning against the king. But on the plus side, once you do take over, ruling is a piece of cake because there are no barons to try to challenge you.
On the other hand, we have places like France, where there's a king and barons that go back a long time. It's super easy to get into one of these places, since a baron somewhere is probably pissed at the king and wants him gone. The problem is, those same barons can turn against you after the old king is gone, so it's hard to hold on to these sorts of kingdoms.
Back to Alexander. Why was it easy to keep his lands? You guessed it: they were like Turkey and had no barons to complicate things.