I read this paperback sci-fi in the early nineties. I have a jumble of memories but not sure of the running order in the book.
Two worlds, one slave-owning, grim and brutal; the other pastoral but high-tech nucleus.
On the "grim world" they use the near-human natives as slaves and are very strict with them, the young human protagonist's mother, as a girl, witnesses her father whip her native playmate to death in the kitchen for not playing how she likes. The native's own mother (who works in the house) watches and somehow ensures the floor is forever stained in that area to remind them of her child.
Still on the grim world, they are descended from the IRA and Iranian terrorists and Afrikaaners and their slogans have got intermixed over centuries. The war cry (and mythical Gods) is "Ire, Iron and Voorstrod" (spelling?) but nobody remembers why. They are nowadays religious fanatics with their own strange creed. They boast about how they used to be able to make aircraft fall from the sky. They have a plot to conquer the pastoral planet.
On "pastoral world" they have a massive space station full of giant warbots held in stasis (Enforcement?) to keep the peace in their star colony area. These are regularly maintained by their tech people.
The young protagoninst has now found himself on the pastoral world (a refugee?) and is in love with a girl there. Suddenly an invasion begins - the grim people have managed to usurp control of the warbots and are sending them through wormholes along with troops.
The warbots question anyone they run into with religious questions but because of his upbringing, the protagonist easily answers and is released to later help overthrow the invaders.