The direct metaphor would be racism in South Africa but certainly refugees also work as an oppressed people. By the time story begins, the aliens have already been on Earth long for the usual symptoms of prejudice and xenophobia to take on a life of their own. The earthlings don’t trust the aliens and so an oppressive atmosphere is cultivated towards them. This oppressive atmosphere creates engender feelings of fear which leads to protective actions by the aliens viewed as aggression which only serves to kindle the differences between then and the fires of prejudice, bias, bigotry and discrimination in a way that strongly parallels the disordered racism that eventual grew into the tightly controlled system of Apartheid.