There is very little I remember about this book, I read it maybe thirty years ago.
I think there was a human sphere of space surrounded by a more advanced alien race, slowly over many years the aliens were encroaching into the human sphere and taking over star systems and basically enslaving the people.
I can't remember (and I've thought hard for some days) many more plot details, except for one scene that I think must have horrified me when I first read it.
The aliens take a human girl into their interrogation centre. I think they were looking for clues to trace a rebel. She plays the "do you know who I am?" card and tells them her father is high up on the liaison council and has a lot of power and influence. They march her down to look into a cell.
The aliens have, over generations, selectively bred a human guard type, massively oversized and muscled but with tiny heads and brains. Three of these guards have her 'important' father pinned down in the cell and are taking turns anally raping him. This makes her realise how very little regard they have for humanity.