4-D War

The Black Sun War was a time war fought between the Gallifreyans (and later the early Time Lords) against the Order of the Black Sun, based thirty thousand years in Gallifrey's future.

The first strike of the war, from the Time Lords' point of view, was when a Black Sun agent travelled back in time to attack Omega and Rassilon's from turning the star Qqaba into a power source for their time experiments. The attempt did not succed but did cause the apparent death of the stellar engineer Omega. (DWM: Star Death)

The Time Lords did not know why the Black Sun (whom they had never encountered before the attack) should have wanted to strike at them, and surmised that it was for something they (the Time Lords) had yet to do. (DWM: Star Death, The 4-D War) Years later, at a diplomatic conference, a representative of the Order was murdered by the Sontarans. The murder was blamed on the Time Lords. This provided the motivation for the war's beginnings, as from the Order's point of view, the Time Lords were the ones who struck first. (DWM: Black Sun Rising)

We do not know how the resst of the war transpired except that with the Order of the Black Sun an obscure entity and the Time Lords much more prominent, the Time Lords, at some stage, almost certainly won.

Notes

The name Black Sun War is purely conjectural.