The mueang or "city-states" formed independent polities bound to the regional power of the king in a system known as a mandala. Each city was headed by a city lord or "chao mueang". The mandala formed an important interdependent system of trade and tribute, which was based more on controlling resources and local populations than it was regional territories. Both wars and the production of rice required large scale labor forces. In Southeast Asia it was common practice for an invading army to forcibly move a population to where they may be more accessible for taxation, conscription or corvee labor. War was also an important means of generating wealth via tribute, and it was not uncommon in the mandala system to pay tribute to more than one regional power at a time.