Your sharpest tool is of course your army. They obey every command you give, marching to the ends of the earth if you wish it so, however… soldiers have expectations. They want fame and glory, booty from war campaigns, and strong and wise leadership. Fail to deliver glorious victories, fail to provide money or supplies, lose their trust in your infallibility, and things can turn bad in a short while.
It is up to you to keep the morale of your armies, their battle-readiness and loyalty, high. Fail to do this and your empire will crumble as if under an earthquake.