There was thus, from the first, a curious interdependence of pope and emperor. No one could be emperor unless crowned by the Pope in Rome; on the other hand, for some centuries, every strong emperor claimed the right to appoint or depose popes. The medieval theory of legitimate power depended upon both emperor and pope; their mutual dependence was galling to both, but for centuries inescapable.

Bertrand Russell, History of Western Philosop…, loc. 1526