The nadir of Portuguese fortunes in Mozambique came in the 1830s and 1840s when Lourenço Marques was sacked in 1833 and Sofala in 1835. Zumbo was abandoned in 1836 and the Gaza Empire forced Afro-Portuguese settlers near Vila de Sena to pay tribute. Although Portugal claimed sovereignty over Angoche and a number of smaller Muslim coastal towns, these were virtually independent at the start of the 19th century. However, after Portugal renounced the slave trade, these towns continued the practice. Fearing British or French anti-slavery intervention, Portugal began bringing these towns under stricter control. Angoche resisted and fought off a Portuguese warship attempting to prevent slave trading in 1847. It took a military expedition and occupation in 1860-1 to end Angoche's slave trade. Portugal also initiated the Prazo system of large leased estates under nominal Portuguese rule in the Zambezi valley. By the end of the 18th century, the valleys of the Zambezi and lower Shire River were controlled by a few families who claimed to be Portuguese subjects but who were virtually independent. However, starting in 1840 the Portuguese government embarked on a series of military campaigns in an attempt to bring the prazos under its control. Portuguese troops suffered several major setbacks before forcing the last prazo to submit in 1869.