The Empires of the past fueled themselves on the drive of conquest and glory, things mirrored in the twentieth century cataclysms of war, when emotion overode the other senses. What was Tiber's purpose but for conguest and glory in the beginning?
Show me the money! It's the economy, stupid. The golden rule: thems as has the gold makes the rules.
Seriously, financial advantage, increased material resources, control of the means of production, drives both people and empire far further than abstract concepts like conquest for its own sake, ever-fleeting glory, or ephemeral emotion. What use is that stretch of land, the title of ruler, or being king of the hill, if it doesn't bring riches to the peasant, craftsman, vassal, and lord?
Every government rises and falls on its ability to distribute the riches that the people fought and died for. Grecian democracies died not from military threats, but when people voted their own self interest out of the treasury. The most serious acts of rebellion usurp this authority, whether by dumping tea into Boston Harbor, forcing new states to decide on freedom or slavery to support an agrarian economy against industrialization, marching to the sea shore to gather your own salt, or establishing a trade embargo against Imperial goods. Lands conquer other lands (stompy robots aside) because they pay for arms, training to use them, transport to get them there, and an entire civilian military-industrial complex to keep the supply train running.
The essence of empire is economic: who gets to regulate and especially tax trade. This is also the bargain provinces make with empire: protect me against enemies foreign and domestic with your cut of our wealth. Fail this, as the Septim Empire did in the provincial wars during the Simulacrum, and again during the Daedric Invasion, and provinces shop around for their own defense forces and refuse to give tribute. If enough provinces do this it can undercut Legion funding and loyalty, reducing the cost of disobedience. Eventually provinces cut back defense costs in the lull between wars, scaling back their standing army instead of funding permanent legions, ornate asylums for mad emperors, and disastrous public-works projects like roads and cities in the Black Marsh. This of course lasts only till Akavir invades again, or the Aka wakes up, whichever comes first.