3 ideas
Moving a society's real traumas onto an invented planet with invented peoples lets a writer depict invasion, collaboration and state violence without naming the regimes or events involved. Readers see the political mechanisms clearly because they are not defending real-world partisan identities. The alien setting also works as cover under censorship.
The most lasting damage of colonisation is internal division rather than military defeat. Occupied people split into collaborators, accommodators and resisters, and resistance factions often turn on one another over legitimacy and leadership. Once that happens, the colonised society helps carry out its own subjugation.
The trilogy presents political violence as recurring in cycles. Each group that is conquered or rebels tends to reproduce the domination it suffered once it gains power. The cycle continues because each generation loses the memory of earlier conquests and their costs.