Such efforts faced drastic limitations. The most formidable was the narrow margin of surplus produced by hand labor in the forests, on the fields and in the workshops, operated, in the main, with hand tools, with minor inputs of energy supplied by domestic animals and with the small amounts derived from wind and moving water. Two further limitations existed. First, as each civilization matured its leaders and policy makers ceased to labor on the land or in the workshops, preferring to keep their hands and clothes clean, to free themselves from irksome demanding toil and devote themselves to tasks more befitting "gentlefolk." This was notably true of landlords as a class. It was also true of the richer traders, merchants and moneylenders, particularly of the third and fourth generations. Expansion of empires and the civilizations which they developed entailed military operations. Military operations, in their turn, produced war-captives, who must earn their keep and, if possible, something more. Sold in the market to the highest bidder, war captives and their descendants became chattel slaves. As civilizations were expanded by conquest and matured by struggle, they developed some type of forced labor to balance the increased parasitism of the masters and the growing numbers who were called upon to produce "services" rather than material goods. Certain areas of civilized economies were taken over by the public authorities. Planning and building of cities and their ports, of highways, including bridges, of viaducts, aqueducts, of drainages for the cities, of public buildings. The construction of defenses, including city walls, were partly or wholly public enterprises. Temples and tombs for the mighty were often in the same category. Maintenance of large elaborate households by political leaders, and in later periods of empire building, by the successful merchants and technicians, led to the employment of many servants, including subordinate members and relatives of the elite. Much necessary labor was performed by members of each household. The resulting economy was therefore fragmented at the household level with virtually all of the energy supplied by human beings and domestic animals. As each civilization developed its pattern of forced labor, including the labor of war captives, it launched the deadly competition between freemen and slaves which almost inevitably ended in favor of the slaves, who were housed and fed by the masters and who could operate at overhead costs lower than those involved in the hiring of wage or salaried workers. Land ownership tended to center in the political-military leaders, the temples and, as each civilization matured, in the hands of its bourgeoisie.