Why can South Africa be described as a single, though not unified, country?

South Africa is one country, but it is not united.  The
country has always been divided by race and this has not ended with the end of the
apartheid regime in the 1990s.

We can see this problem in
current events from South Africa.  The African National Congress has been in power since
1994, but not everyone is pleased with the way it has governed.  It is now challenged to
some extent by the Democratic Alliance, led by a white woman, Helen Zille, who was an
anti-apartheid activist.  This challenge has highlighted and exacerbated racial
tensions.  The DA's membership is mainly among whites and coloreds.  ANC officials are
portraying any opposition to the ANC as an act of
racism.

This sort of thing shows how far South Africa is
from being a united country.