In fiction, an interstellar war is a war between combatants whose respective headquarters lie in different planetary systems. It is a popular plot device in science fiction, especially in the space opera subgenre. An intergalactic war refers to war between combatants of different galaxies. Interplanetary war refers to war between combatants on different planets of the same system.
The literature on interplanetary war is often revealing of contemporary mores at the time it was written. In general, older fiction relates to the colonial systems of politics and economics of the late 19th/early 20th centuries; mid-20th century depictions are often heavily influenced by the Cold War and are often barely concealed analogies of the conflict between the "free world" and the Communist states, with Earth (personified in 1950s-style American archetypes) as the "good guys" and aliens as the "bad guys". Modern fiction often uses the conflict to explore known failings in the human (especially Western) perspective.
The earliest fictional references appear to be to interplanetary, not interstellar war (e.g. H. G. Wells' The War of the Worlds). Now that the other planets of the solar system are believed to be devoid of intelligent life, sci-fi writers generally posit some form of faster-than-light drive in order to facilitate interstellar war. However, writers such as Larry Niven have developed plausible interplanetary conflict based on human colonization of the asteroid belt and outer planets using only technologies using the laws of physics as currently understood.
Notable examples of Interstellar/interplanetary War in fiction: