The California missions comprise a series of religious outposts established by Spanish Catholic Dominicans , Jesuits , and Franciscans , to spread the Christian doctrine among the local Native Americans . Eighty percent of the financing of Spain's California program went not to missions but rather to the military garrisons established to keep Britain and Russia out of the three great Pacific Ports of San Diego, Monterey and San Francisco; the Santa Barbara presidio on the Channel was added later. The primary benefit to Spain of occupying California was to confirm historic claims to the territory in the context of imperial war with Britain. The missions introduced European livestock, fruits, vegetables, agricultural industry, along with invasive species of plants into the California regions. It is widely believe in California that the labor supply for the missions was supplied by the forcible relocation of the Native Americans and keeping them in peonage .  However, more recent scholarship suggests that the tiny number of Spaniards at each mission relied more upon negotiation, enticement and the threat of force to control the estimated 5,000 Indians typically surrounding what would become a mission. The missionaries and military were often at cross purposes in their vision of what California could become, and the missionaries preferred to rely upon Indian allies to maintain control.