As far as I can gather (as this is one of my least favourite sub-genres of science-fiction) Military Sci-Fi uses futuristic technology and weapons, often to negotiate fights with alien civilisations, or conduct battles in space, although I’ve read many a book where the military is used in a present day setting, usually with a near apocalyptic scenario.
Books in this genre appear to have main characters who are, in fact, military personnel, which means that I’ve been writing a novel in a genre I claim not to be keen on (my novel draft has military personnel and most of the story take place on military premises – who knew?)
Space opera is a subgenre of Science Fiction (and a form of Military Sci-fi) involving romance, melodrama and is set mostly in outer space according to the Wikipedia page on the subject. Clearly I’ve not understood this genre at all (until now), as a lot of television shows I enjoyed as a child, and which instilled me with a life-long interest in science fiction, appear to fall under this category: Battlestar Galactica, Doctor Who and Blake’s Seven to name but a few.
So, all that talk about this genre being my least favourite means I’ve been clearly misinformed at some point in my life. It’s a genre I read a lot of, and a genre I have chosen to write in, a genre I love in fact…