Three pillars supported the empire of New Spain. The first two, the presidio and the mission, have lived on in history and the popular imagination. The third, less studied and less understood, has lived on in the traditions of local self-governance and the distinctive cultural and social patterns of the Southwest. That third pillar is the civil settlement, or town, with its distinctive governmental institutions.

Town councils, or cabildos, brought to the northern frontier a high degree of law and order, patterns of local government, a rough democracy, and the principle of justice based...