Cordelia's Honor by Lois McMaster Bujold

A capable Betan survey captain is drawn into the treacherous politics of a proud, warlike planet when she becomes entangled with a principled Barrayaran officer; amid conflict, blockade and betrayal their professional respect deepens into marriage as they confront assassination plots, military treachery, and stark cultural clashes. Together they struggle to protect one another and their family while using wit, courage, and moral steadiness to steer a fragile nation through crisis and reshape the future of a society on the brink.