Belgium participated in the Industrial Revolution and, during the course of the 20th century, possessed a number of colonies in Africa. Between 1888 and 1908, Leopold II, king of Belgium, perpetrated one of the largest massacres in human history in Congo Free State, which was his private estate, and not yet a colony of Belgium. Estimates of the death toll are disputed but millions of people, a significant part of the population, died for the sake of rubber and ivory exports.