The Prince by Niccolo Machiavelli Chapter 4 Page 7

each to carry with him some portion of these Provinces, according as each had acquired influence there; for these, on the extinction of the line of their old Princes, came to recognize no other Lords than the Romans.

Bearing all this in mind, no one need wonder at the ease wherewith Alexander was able to lay a firm hold on Asia, nor that Pyrrhus and many others found difficulty in preserving other acquisitions; since this arose, not from the less or greater merit of the conquerors, but from the different character of the States with which they had to deal.