The way to withdraw is the way that Julius Caesar withdrew from conquered principalities in Gaul. After defeating some tribe or other he'd kill the leaders and put the strongest surviving noble family from an adjoining kingdom or tribe in charge. Then he'd garrison a small force of legionnaires by the capital town to support the weak new guy. This created a patchwork of weak local states each dependent on the local centurion for his authority.

Another way to do it is also a strategy of Caesar's, also employed in Gaul. You conqure a people, kill the leaders (or take family hostages, which you kill if they new folks fork-up) then put somebody new in charge of the conquered tribe and then leave. If it doesn't work out you again invade and kill the leadership and establish a new political family as leader. Then shake your finger at the new family and say, "Make it work or I'm coming back for you and your family and friends".