UNCN ships participated in the Second Inner Planet War, one of the last space-based wars fought among humans. In 2191, the Lunar Inner Planet Fleet launched toward the outer solar system to intercept unidentified alien vessels that had been detected from Uranus. Under orders from Central Command on Earth, the Cosmo Navy ships opened fire on the Garmillas vessels without warning or provocation, triggering a long conflict that would drive humanity to the edge of extinction. Despite the technologically inferior Cosmo Navy's costly but successful efforts at the Second Battle of Mars, the enemy continued to bombard Earth, slowly destroying the planet's ecosystem and draining resources available for fighting ("Messenger of Iscandar", "A World I Once Saw").