This subgenre of science fiction focuses highly on the elements of military warfare. These stories usually involve wars between within a planet or wars among different planets. There is a huge emphasis on advanced technological weapons which include laser guns and battleships.
A very famous example of military science fiction would be H.G. Wells' "War of the Worlds". The story revolves around the invasion of Martians during 1894 and is told by 1st person narration as a character who is directly involved in the action. The Martians use various alien technologies such as tripods, heat rays and flying saucers.
Another famous piece of work in this subgenre would be Heinlein's "Starship Troopers". This plot of this story also entails the an interstellar war between humans and the Arachnids from Klendathu. This story examines many political and military themes.