Complex sentence: When the National Party won the 1948 election in South Africa and subsequently introduced apartheid legislation, these laws also extended into Southwest Africa which was the "de facto" fifth province of South Africa.
Split into simpler sentences: When the National Party won the 1948 election in South Africa, it passed laws establishing racial segregation known as apartheid. It applied these laws to Southwest Africa as well, which it governed as the "de facto" fifth province of South Africa.