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.
Early time wars of Gallifrey
The Time Wars were a collections of time wars fought between the early Time Lords and other races that were developing time travel, (PROSE: Sky Pirates!) lasting 30,000 years. (PROSE: The Infinity Doctors) 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!)
Among the Time Wars was the Black Sun War, (PROSE: Heart of TARDIS) 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)
According to one account, however, before the Last Great Time War only two other time wars had ever taken place — both of each started by other species, but ended with relative ease by the Time Lords stepping on. One was the "skirmish" between the Halldons and the Eternals, and the other being was the "brutal slaughter" of the Omnicraven Uprising. (PROSE: Meet the Doctor)
The Doctor's era
When visiting the planet Aoris, the Fourth Doctor, Romana II and K9 discovered that the planet was engaged in a time-travelling war as residents of Era 24 attacked the 'Age of Greed' in Era 14, approximately a thousand years before their time, stealing endangered animals and the potent natural power source of zenox crystals from the past in the belief that their ancestors were incapable of using these resources properly. In the course of his time on Aoris, the Doctor realized that this war was the result of a complex time loop, as the people of Era 14 only came to rely on more environmentally-dangerous power sources such as fossil fuels because the zenox crystals were stolen by their future descendants. The Doctor, Romana and K9 were eventually able to end hostilities between the two sides, leaving a few people from Era 24 in Era 14 to set up a resistance movement that would aid them in Era 24 while sending the rest of Era 24 into the further future so that Aoris could be regenerated by various terraforming engines (AUDIO: The Paradox Planet/Legacy of Death).
In the earlier stages of their history, the Daleks made some attempts to weaponise time travel in war. In the 26th century the Daleks sought to escalate the Second Dalek War into a temporal conflict by utilising time travel to defeat the Earth Empire. They were overambitious and unsuccessful, contributing to their ultimate defeat, but the research laid the groundwork for future campaigns of the same nature. (PROSE: Prisoner of the Daleks, AUDIO: Out of Time) The Daleks later worked with renegade Time Lord Shazar to attempt to make a fleet of time machines, but were foiled by interference of the Fourth Doctor and the Time Lord High Council. (COMIC: Return of the Daleks) The Daleks would eventually develop their own form of time machine, which deployed to hunt down the First Doctor on two occasions; firstly in a failed execution attempt, (TV: The Chase) and in the 40th century to invade Earth in 2415.
However, the Dalek Emperor's personal time machine proved unreliable and he was separated from his force, which was defeated. (COMIC: The Secret of the Emperor) In 4000 a time machine was used by the Daleks during the Time Destructor Incident to retrieve the taranium core stolen by the First Doctor. (TV: The Daleks' Master Plan) In 4590, (PROSE: Resurrection of the Daleks) the Daleks employed a time corridor during their rescue of Davros, embroiling 1984 London in events occurring centuries later. (TV: Resurrection of the Daleks) During the Imperial-Renegade Dalek Civil War, two rival Dalek factions travelled back to 1963 to claim the Hand of Omega, resulting in the Shoreditch Incident. (TV: Remembrance of the Daleks) The Daleks would later use time travel technology in their successful gambit to destroy and rebirth the Seriphia Galaxy. (AUDIO: The Apocalypse Element)
Following the Daleks' attempt to invade Gallifrey itself according to one account, (AUDIO: Acension) the Time Lords sent the Fourth Doctor back in time in an attempt to avert the Daleks from evolving. (TV: Genesis of the Daleks) In doing so, they fired the "first shot" of a later Last Great Time War. (COMIC: Hunters of the Burning Stone)
War in Heaven
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)
Last Great Time War and beyond
Pa-Jass Vortan was a Dalek phrase that translated to "the Time War." (PROSE: The Slyther of Shoreditch) According to the Seventh Corsair, the Destruction of Skaro by the Hand of Omega triggered a "little" time war, which was eventually erased by the Last Great Time War. (PROSE: One Virtue, and a Thousand Crimes) In this much larger war, the Time Lords fought against the Dalek Empire, which multiple incarnations of the Doctor ended by freezing Gallifrey in a pocket universe, making the Daleks accidentally destroy one another in their own crossfire, while the rest of the universe believed that both sides had just annihilated each other. (TV: The Day of the Doctor)
The Kovarian Chapter waged a war through time against the Eleventh Doctor, having gone back along his timeline in an attempt to avert his arrival at Trenzalore. Tasha Lem warned the Doctor that if the Time Lords returned to the universe then the species waiting above Trenzalore — among them the New Dalek Paradigm, 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 fourteenth incarnation. (TV: The Time of the Doctor)
With the aid of Rassilon as Cyber-President, the Cybermen launched an all-out war throughout history, heralding the Age of the Cyberiad. When the Cybermen betrayed Rassilon, he united with the Twelfth Doctor to prevent the invasion from beginning. (COMIC: Supremacy of the Cybermen)
In anticipation of the Flux, the Sontarans waged a Temporal Offensive on Earth, sending timeships back through time and overwriting the planet's history. This offensive was ultimately thwarted by the Thirteenth Doctor and her companions, averting the changes the Sontarans had made. (TV: War of the Sontarans)
In the Unbound Universe, the Great War was a time war that ravaged the cosmos. The fighting came to a close when the Master unleashed a superweapon that destroyed most of the universe. (AUDIO: The Emporium At The End) After the War, no-one could recall what had started it. (AUDIO: The Library in the Body)
In the Warrior's universe, a Time War began after the Warrior's interference in the creation of the Daleks, (AUDIO: Dust Devil) as his actions led to the formation of the Unified Skaroan Alliance who developed time travel from the time ring he'd left behind and then sought to control the universe. (AUDIO: Aftershocks)