A long-range scout of an alien race exploring a star system finds a planet suitable for colonisation and encounters an Earth explorer-class cruiser. The cruiser's AI caused it to jump into the system, so the encounter isn't expected by either the alien or the humans on the cruiser.
Misunderstanding the cruiser's actions, the alien scout fires on the cruiser, not that its weaponry is capable of doing any damage. The cruiser jumps out of the system, exhausting its fuel and never reaching home... leading to a long, long war that lasts centuries.