 It is now generally believed that diseases, such as the smallpox, brought upon by the Columbian Exchange, were the greatest destroyer, although the brutality of the conquest itself isn't contested.Genocidal policies were more common in post-colonial states, notably in 19th century United States and Argentina where indigenous populations were systematically exterminated.For example, Juan Manuel de Rosas, Argentinian "caudillo" from 1829 to 1852, openly pursued the extermination of the local population, an event related by Darwin in "The Voyage of the Beagle" (1839).