Encomienda is a Spanish word related to the Spanish verb <em>encomendar </em>which means <em>to entrust, </em>and that is the key of the Encomienda. It was a labor system was in which victorious conquerors were assigned to manage land an workforce (indigenous and slaves) as vassals of the Spanish Crown. They could work the land, grow crops and collect taxes of the region as representative of the Crown. That is why we can say the conquistadores were trustees during the encomienda system.
I suppose it made the free states and slave states in some way even. When Missouri became a slave state they kept the north end of the Louisiana purchase free stated and banned slavery there.
