To put it in a terminology that hearkens back to the more brutal age of ancient empires, the three grand imperatives of imperial geo-strategy are to prevent collusion and maintain security dependence among the vassals, to keep tributaries pliant and protected, and to keep the barbarians from coming together.
The Grand Chessboard: American Primacy And Its Geostrategic Imperatives (ed. Basic Books, 2016) - ISBN: 9780465094356