In the story Once Upon A Time, what is the "truth" that Gordimer develops throughout the story?

In Once Upon A Time by Nadine Gordimer, Gordimer develops the story of a family which goes to ever-increasing lengths to protect itself from an unknown enemy in the mistaken belief that it is "the people of another color" whom they should fear. This fictional short story reveals the sad truth attached to social and political ignorance. In South Africa in 1989 when the story was originally published, the political landscape was changing and some sectors of South African society were afraid of the repercussions from an unjust and racially-divided system (apartheid) which was crumbling.

Ironically, the more steps this family takes to separate itself from "unemployed loiterers" who make the man and his wife uncomfortable, and whom they think spoil "this beautiful suburb", the more the man and his wife become blinded by the real threat of their own lack of awareness and their own contribution to an intolerable situation.

The family in this story creates and intensifies its own fear; there is no malicious intent. Gordimer stresses that this family is not overtly "racist" and the wife feels a certain responsibility towards the less fortunate people who she sees but never interacts with, hating "to see anyone go hungry", although the reader is not entirely sure whether it is her own guilt or her genuine compassion which leads to her actions. She is easily persuaded that building the wall higher is the best option. Even the "trusted housemaid" has unwittingly become part of this system as she sees only "loafers and tsotsis" outside, revealing the absurd nature of the system and its harmful and divisive effects (a truth that many people refuse to accept). 

In real life, the system (Apartheid) is collapsing, and in the story, the man and his wife refuse to accept any change which makes them uncomfortable, even though it is that very system that has contributed to, and now builds on, their fears. Gordimer reveals the truth that it is not the "people of another color" who have created the fear, but the institution. Now that it is changing, the poor response to it generates it own extremes. The tragedy which occurs is unrelated to the apparent threat from outside and the story reveals that there is no concept of what the underlying problem is.