By the 1920s, Lemkin was formulating the concepts and laws that were articulated in his best-known book, Axis Rule in Occupied Europe (1944). His unpublished manuscripts reveal that he saw colonialism as an integral part of a world history of genocide.
Those manuscripts covered an extremely wide range of cases where European colonial powers were responsible for mass killing, from the Spanish conquest of the Americas in the sixteenth century and the slaughter of indigenous peoples in Australia and New Zealand to the German massacre of the Hereros in Namibia a few decades earlier.