By the late 16th century, Bolivian silver was an important source of revenue for the Spanish Empire. A steady stream of natives served as labor force under the brutal, slave conditions of the Spanish version of the pre-Columbian draft system called the mita. Charcas was transferred to the Viceroyalty of the Río de la Plata in 1776 and the people from Buenos Aires, the capital of the Viceroyalty, coined the term "Upper Peru" () as a popular reference to the Royal Audiencia of Charcas. Túpac Katari led the indigenous rebellion that laid siege to La Paz in March 1781, during which 20,000 people died. As Spanish royal authority weakened during the Napoleonic wars, sentiment against colonial rule grew.