Originally it was occupied by South African troops in 1915 as part of WWI. Namibia was a German colony called German South West Africa and South Africa committed itself to fight with the British side of the war effort. While the South African government wanted to annex the territory the major powers wouldn't allow for an out-and-out annexation but created it into a League of Nations mandate for South Africa in 1919-1920 (I've seen both). Even after WWII South Africa maintained its claim despite UN and other international objections. White settlers did begin to settle there and the racial policies of what would become Apartheid followed (the Germans had also had racial segregation policies in place prior). South Africa tried to incorporate it as a fifth state but did not in the face of international pressure.

These articles go into greater detail: http://www.content.eisa.org.za/old-page/namibia-south-african-mandate-1915-1945 http://www.namib.info/namibia/uk/history/resistance_against_south_africa/index.php http://thecommonwealth.org/our-member-countries/namibia/history

(also, this is my first response on /r/askhistorians so be gentle if I broke any rules!)