The Dawn War was a galaxy-spanning conflict, encompassing even its two satellite galaxies. It took place roughly one billion years before humanity's first interstellar flight.
To varying degrees of involvement, several thousand species took part in the war. Though short compared to the billions of years of priming it took to ignite, the Dawn War lasted long enough for whole races to evolve into several splinter species; for organic civilisations to transform themselves into machines, and then return to their original forms when their nostalgia for the ancient past overwhelmed their desire for efficiency.
At the war's conclusion only one group survived: the Inhibitors. Whether a single species, or an alliance of them, even they no longer remembered. A hybrid organic-synthetic race, they would dominate the galaxy for the next billion years.
Evidence of the destruction caused by the conflict survived even to the time of humanity's rise, if one knew where to look.