With the establishment of European settlement and its subsequent expansion, the Indigenous populations were progressively forced into neighbouring territories, or subsumed into the new political entities of the Australian colonies. Violent conflict between Indigenous peoples and European settlers, described by some historians as frontier wars, arose out of this expansion: by the late 19th century, many Indigenous populations had been forcibly relocated to land reserves and missions. The nature of many of these land reserves and missions enabled disease to spread quickly and many were closed as resident numbers dropped, with the remaining residents being moved to other land reserves and missions in the 20th century.