Causality paradox
A causality paradox is a phenomenon in temporal mechanics in which history changes with the erasure of a species' historical existence. If this happens, a new timeline is created in which all of the history in which the species played any part whatsoever, directly or indirectly, is altered. This effect occurs because, since the species never existed, the influence it would have had if it had existed never occurred.

In multiple alternate timelines, this phenomenon was caused several times by a weapon known as the Krenim weapon ship. This weapon was built by the brilliant Krenim military temporal scientist Annorax. It caused causality paradoxes through the use of its primary weapon, which was capable of erasing a species from history by firing an energy beam on the species' homeworld. The beam pushed the species out of the space-time continuum, effectively erasing it from history. Between 2174 and the end of 2374, the weapon was used on several species and caused ensuing causality paradoxes. The following are known uses of the weapon, along with the causality paradoxes that occurred:
 The erasure of the Rilnar. The Rilnar had removed the Krenim from their place of power in their region of the Delta Quadrant. Thus their erasure caused a causality paradox in which the Krenim stayed in power since the Rilnar never existed to remove them. Unfortunately, this also led to the near destruction of the Krenim themselves, since Rilnar physiology contributed a crucial antibody to Krenim physiology. Thus the erasure of the Rilnar also caused a causality paradox in which a disease, which the antibody would have prevented the Krenim from getting, killed fifty million Krenim in a year.
 The erasure of the Zahl. In one of the weapon ship's created timelines, the Zahl, not the Rilnar, removed the Krenim from power. Thus, their erasure caused a causality paradox in which the Krenim were never removed from power. This also led to the near destruction of the Federation starship , which was passing through Zahl space at the time, and so, in the new timeline, was therefore passing through Krenim space and was incessantly attacked by Krenim warships as a trespasser.
 The erasure of the Ram Izad. This caused a causality paradox that saw the restoration of Krenim control over approximately half of the territory they had controlled before.

Aside from these instances, the Krenim temporal weapon ship was used to cause other causality paradoxes by the erasure of two other known races: the Malkoth and the Alsurans. The exact effects of the causality paradoxes caused by these erasures, however, are unknown. Finally, near the end of 2374, all of these causality paradoxes were eliminated when the weapon ship's temporal core, which gave it its power to cause temporal incursions, was destabilized by a suicide run made by the USS Voyager. This caused a temporal incursion on the ship itself, erasing it from history, and reversing the causality paradoxes it had caused. ().

Physics