A time war could be one of at least two types of time-spanning conflicts. The first type of time war was where sides fought each other across different points in history. The second type of time war was one in which time itself was used as a weapon by two or more time-active factions, employing preemptive strikes, time loops, temporal paradoxes and the reversal of historical events. It is difficult to study these wars, since they tended to erase the damage before it was made.
The first time war fought by Gallifrey was the Black Sun War, starting when the Order of the Black Sun sent a time-travelling assassin to prevent Gallifrey from gaining time travel. (COMIC: Star Death, 4-D War) The Order itself existed 30,000 years in the future, and launched a second attack in the Question Hall twenty years after the initial attack. (COMIC: 4-D War) The Time Lords had been viewed by the Order as striking first in the present, after the Sontaran ambassador Brilox used a psy-snare to make the parahuman Millenium assassinate the Black Sun Elder in an attempt to secure the uranium on Desrault for the Sontarans while Gallifrey and the Black Sun were "at each other's throats". (COMIC: Black Sun Rising)
A second war, lasting thirty thousand years, took place a generation after the time of Rassilon. It was fought between the Time Lords and other races that were developing time travel. The Time Lords destroyed one such race, the Charon, before they even existed. Another unnamed species which was eliminated created the reality bomb. (PROSE: Sky Pirates!)
At one point in the Eighth Doctor's future, the War in Heaven was fought between the Time Lords and the enemy. (PROSE: Alien Bodies) The Eighth Doctor destroyed a Gallifrey while attempting to prevent this war from beginning. (PROSE: The Ancestor Cell)
Sutekh and Horus agreed that the Osirian throne would go to the winner of a time war. (AUDIO: Ozymandias) The resultant conflict was orchestrated by Justine so both Sutekh and Lolita would be defeated. (AUDIO: The Judgment of Sutekh)
The Time Lords fought against the Daleks in the Last Great Time War, which "all thirteen" incarnations of the Doctor ended by freezing Gallifrey in a pocket universe, making the Daleks accidentally destroy one another and it appearing to the rest of the universe that the Time Lords and the Daleks had destroyed each other. (TV: The Day of the Doctor)
Tasha Lem warned the Eleventh Doctor that if the Time Lords returned to the universe then the species waiting above Trenzalore — among them the Daleks, Cybermen, Sontarans, Terileptils, Judoon and Weeping Angels, along with the Slitheen family — would attack them and the Siege of Trenzalore would escalate into another time war. The Daleks managed to break through the Papal Mainframe's force field, resulting in a war on Trenzalore and many of the other fleets were destroyed or retreated overtime. After 900 years of stalemate and with the Doctor dying of old age at the end of his final incarnation, the Time Lords granted him a new cycle of regenerations, with the resulting regenerative "reset" destroying the Dalek forces, ending the Siege, averting a new Time War, and beginning the Doctor's thirteenth incarnation. (TV: The Time of the Doctor)