The massacre of modern-day Namibia's indigenous Herero and Nama groups by German imperial troops more than a century ago is considered by some historians to be the first genocide of the 20th century.As Germany prepares to return some of the victims' bones on Wednesday, here is some background.RebellionGermany ruled what was then called South West Africa as a colony from 1884 to 1915.Angered by German settlers stealing their women, land and cattle in their remote desert territory, the Herero tribe launched a revolt in January 1904. Its warriors butchered 123 German civilians over several days.The smaller Nama tribe joined the uprising in 1905.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookExtermination orderThe German occupiers responded ruthlessly, defeating the Herero in a decisive battle at Waterberg, northwest of the capital city of Windhoek, on August 11, 1904.German troops then pursued them as some 80 000 fled towards Botswana, with women and children, across what is now known as the Kalahari Desert. Only 15 000 survived.In October 1904 German General Lothar von Trotha, under the direct command of Kaiser Wilhelm II in Berlin, signed a notorious "extermination order" against the Herero."Within the German boundaries, every Herero, with or without a gun, with or without livestock, will be shot dead," he said.Survivors were sent to concentration camps, decades before those in which millions of Jews and others were exterminated during World War II.An estimated 60 000 Herero and 10 000 Nama people were killed from 1904 to 1908.From 40% at the start of the 20th century, the Herero now only make up seven percent of the Namibian population.Bones for 'experiments'Hundreds of Hereros and Namas were beheaded after their deaths and their skulls were spirited away to German researchers in Berlin for since discredited "scientific" experiments to prove the racial superiority of whites over blacks.In 1924 a German museum sold some of the bones to an American collector, who donated them to New York's natural history museum.In 2008 Namibia's ambassador in Berlin demanded that the bones be returned, saying it was a question of reclaiming "our dignity".Germany has since 2011 formally handed back dozens of the skulls, many of which were stored at universities and clinics.Demand for apology, reparationsGermany long refused to take the blame for the episode, only accepting responsibility on the 100th anniversary of the massacres in 2004, when the then development minister, Heidemarie Wieczorek-Zeul, said the "atrocities... would today be called genocide".Some historians and other officials have also termed the massacre a genocide, including then German parliamentary president, Norbert Lammert, in 2015.The German government said in 2016 that it planned to issue a formal apology, but negotiations are still ongoing.Berlin has also refused to pay reparations to descendants of the Nama and Herero victims, arguing that its development aid worth hundreds of millions of euros since Namibia's independence from South Africa in 1990 was "for the benefit of all Namibians".Herero and Nama representatives have filed a class-action suit against the German government in a US court to demand damages. The New York judge has yet to rule on whether to hear the lawsuit, which Germany wants thrown out.