Can Long Endure (The End of All Things #3, Old Man's War #6.3)

Part three of the four parts making up the full-length novel, The End of All Things.
They signed up to defend humans from hostile aliens, but this group of Colonial Union soldiers finds themselves, instead, repeatedly sent to squelch rebellious human colonies that want to leave the CU. It's not a sustainable situation. Something has to give. The End of All Things is John Scalzi's conclusion to the Old Man's War tale that began with The Human Division.