...As late as the 1870s, only 10% of the continent was under direct European control, with Algeria held by France, the Cape Colony and Natal (both in modern South Africa) by Britain, and Angola by Portugal. And yet by 1900, European nations had added almost 10 million square miles of Africa- one-fifth of the land mass of the globe to their overseas colonial possessions. Europeans ruled more than 90% of the African continent... -Saul David, "Slavery and the 'Scramble for Africa,'"

BBC British History in Depth

Which event most directly influenced the change referred to in this passage