Force Empire Core Strength (manual)

Force is the radiant power that drops off most slowly with the lowest potential yield for any given system. If your empire's at-distance force is greater than a system's, you will occupy that system. As one would expect, occupation is the most primitive kind of relationship so it has the weakest possible outcome.

Occupation gives you a small wealth boost but absorbs a great deal of force in the process.

Force is also required to suppress piracy or resistance. Your trade and cultural potentials at any given point cannot exceed your force at that point. This makes occupation a far more expensive prospect than it otherwise would be, as it limits your ability to trade with or include further systems.