Oppression of Indigenous Australians is a prominent theme in Australian history. Despite encountering Indigenous Australians upon their arrival, members of the First Fleet invoked the principle of "terra nullius" to claim the continent. This was followed by a long period of recurrent massacres and violent conflicts. By the 20th century, the Australian government adopted a policy of forcibly separating mixed-race Indigenous children from their families, which remained in place until the 1970s.