Temporal incursion

A temporal incursion is any action in which an individual or agency uses time travel in an attempt to change history. Examples of temporal incursions include the actions of Krenim scientist Annorax, and various factions in the Temporal Cold War. As of the 29th century, there were agencies specifically tasked with protecting the timeline from temporal incursions, such as the Temporal Integrity Commission, and the temporal agents of the 31st century.

Temporal Cold War
Numerous temporal incursions occurred as part of the Temporal Cold War. Multiple factions, each one vying for control, attempted to change history to suit their own ends. Examples include the Suliban Cabal's mysterious benefactor attempting to sabotage the 's mission, the Sphere Builders attempting to manipulate the Xindi into destroying Earth, and Vosk collaborating with the Nazis in 1944. These factions were opposed by Temporal Agents, who represented themselves to the crew of Enterprise in the form of Daniels, who would often assist the crew in protecting the timeline. 

The Borg
In 2373, a Borg sphere traveled back in time to April fourth, 2063, in an attempt to prevent Humanity's First Contact with extraterrestrials and assimilate the Human race at a time period in which they were ill-equipped to offer resistance. Arriving in Earth orbit through a temporal vortex, the sphere immediately began firing upon Zefram Cochrane's launch facility in central Montana, but was itself destroyed by the crew of the USS Enterprise (NCC-1701-E), who had followed the Borg through the vortex. The crew subsequently assisted Cochrane in making repairs in time for his scheduled launch, ensuring that a Vulcan survey ship detected his warp signature, making First Contact on April 5th. 

The events of this incursion constituted a Pogo paradox, in which interference to prevent an event actually triggers the same event. It also set into motion a chain of events in which a group of surviving drones were revived in 2153, and subsequently sent a message to the Delta Quadrant, where it was received by the Collective in the 24th century, alerting the Borg as to the location of Earth. 

Annorax
Annorax, a Krenim scientist, initiated a multitude of temporal incursions during his quest to restore the Krenim Imperium to power. Using a massive weapon ship to literally erase objects from history, he attempted to manipulate the timeline in order to ensure Krenim superiority. His efforts met with failure, however, when erasing the Rilnar civilization from the timeline had disastrous effects for the Imperium. For over a century, Annorax and his crew attempted to restore the Imperium by erasing ships, colonies, and even entire civilizations from history. In 2374, Annorax was opposed by the crew of the USS Voyager, who were attempting to cross through Krenim space on their journey back to the Alpha Quadrant. Eventually, as a result of massive battle damage, a temporal incursion was initiated within the weapon ship itself, erasing it, and all of the damage it had caused to history, from the timeline. 

Voyager
During its seven-year voyage through the Delta Quadrant, the starship Voyager was involved in numerous temporal incursions, including those involving the Krenim. Some notable examples include:


 * Traveling back in time to Earth, 1996, after a confrontation with 29th century Starfleet officer Captain Braxton, in which Braxton described Voyager's involvement with a devastating temporal explosion in the future. Ironicaly caused by Braxton's efforts to prevent it. Future saved when Voyager destroys Braxton's temporal ship then is returned to its correct time by Braxton from the corrected future.
 * A temporal inversion in the Takara sector, wherein Chakotay and Harry Kim successfully changed history, saving Voyager from destruction in 2375.
 * Stopping Captain Braxton from destroying Voyager in 2375 with a temporal disruptor, and initiating a phaser fight with Seven of Nine at Utopia Planitia in 2371.
 * Meeting a future version of Vice Admiral Kathryn Janeway and receiving future technologies and weapons such as the transphasic torpedo and the ablative generator in order to get home via transwarp hub. This allowed Voyager to get home and destroyed Unimatrix 01 as well as the Borg Queen and the transwarp hub, potentialy crippling the Borg.

Nero
In the year 2387, when a supernova threatened to destroy the entire galaxy, Ambassador Spock utilized red matter to create a black hole with which to absorb the exploding star. Although Spock's efforts were successful, he was too late to save Romulus, which was destroyed. Nero, captain of the Romulan mining vessel Narada, confronted Spock, holding him responsible for the destruction of his homeworld. During the confrontation, both vessels were pulled into the black hole and transported back through time, the Narada arriving in the year 2233, resulting in the creation of an alternate reality. Upon emerging from the black hole, Nero immediately attacked the USS Kelvin, setting into motion a chain of events causing a drastic difference in the histories of the two timelines. Spock, having entered the black hole only moments after the Narada, emerged in the year 2258, and was immediately captured by Nero, who forced Spock to witness the destruction of Vulcan at Nero's hands. Eventually, with assistance from Spock, James Kirk and Spock's alternate reality-counterpart were able to put a stop to Nero's efforts to perform the same action on every Federation world, destroying the Narada with another artifically-created black hole.