The Temporal Wars were a series of conflicts fought throughout time by various factions trying to change the past for their own benefit. (ENT: "Storm Front", "Storm Front, Part II")
At some point, multiple Temporal Wars were fought over the existence of Khan Noonien Singh. As a result, the original events concerning Singh's rise were pushed back, reinserting themselves at a later date in the timeline. (SNW: "Tomorrow and Tomorrow and Tomorrow")
Several Temporal Wars were fought throughout the 30th century. (DIS: "Die Trying") During these wars, many attempted to use the Guardian of Forever as a means of time travel. As a result, the Guardian moved itself far away from its original location, going into hiding on Dannus IV. (DIS: "Terra Firma, Part 1", "Terra Firma, Part 2") During the Temporal Wars, Krenim time bugs were used to paralyze starships by randomly cycling them through time until the bug ran out of power. Following the wars, they were available on the black market. (DIS: "Face the Strange")
Another Temporal War began after the Temporal Cold War "turned hot" when the Na'kuhl returned to the 29th century after making changes to Earth's history, resulting in at least one alternate timeline. This particular Temporal War was negated when Daniels transported the Enterprise NX-01 and Silik to 1944 of the alternate timeline, allowing the crew to prevent the Na'khul from returning to their own time, thus undoing all of the damage that had been done during the war. As the timeline reset itself, Daniels told Captain Jonathan Archer that the Temporal Cold War was coming to an end. (ENT: "Storm Front", "Storm Front, Part II")
After the Temporal Wars, time travel was outlawed completely as a part of the Temporal Accords. (DIS: "Die Trying") Because of this, Starfleet was unable to help Yor or Philippa Georgiou when they became afflicted with a time sickness as a result of being separated from their own time periods and universes. Georgiou and Commander Michael Burnham sought out the Guardian of Forever for help. The Guardian put Georgiou through a test to evaluate how she would affect another universe or timeline. After Georgiou passed the Guardian's test, it sent her back to a time where she could survive. (DIS: "Terra Firma, Part 1", "Terra Firma, Part 2")
In 2384, Wesley Crusher told Dal R'El, Rok-Tahk, Jankom Pog, Gwyndala, Murf, and Zero that with the Temporal Wars, there weren't many Travelers left to fix the dying timelines. (PRO: "The Devourer of All Things, Part I")