Interstellar war
An interstellar war involves a warfare between combatants from different planetary systems. The concept provides a common plot device in science fiction, especially in the space opera subgenre. In contrast, the term intergalactic war refers to war between combatants from different galaxies; interplanetary war refers to war between combatants from different planets of the same solar system.
Interplanetary war in fiction can reveal contemporary mores. In general, older fiction relates to the colonial systems of politics and economics of the late 19th and early 20th centuries; mid-20th century depictions often show a heavy influenced from the Cold War and are barely concealed allegories of the conflict between the "free world" and the Communist states, with humans (personified by 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.
Interstellar war in fiction
The earliest fictional references appear to deal with interplanetary, not interstellar war (e.g. H. G. Wells' 1898 novel 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. Writers such as Larry Niven have developed plausible interplanetary conflict based on human colonization of the asteroid belt and outer planets by means of technologies utilizing the laws of physics as currently understood.