Third Galactic War

The Third Galactic War was an event that occured in the year 5 billion PLF. After the destruction of the Earth by Sol's expansion, the Human Race started a mass-exodus of societal expansion that filled many of the uninhabited planets of the Milky Way. Because of this, some races started enfringing on each other's territory, and some even attempted to force Humans away from planets by force just so that they could then take them. This constant quarrelling soon broke out into all-out war when the Human governments decided that they were not going to be pushed around any more.

Initial Conflict
Despite the warnings issued by Humanity, races like the Quarians, the Sontarans and the Klingons decided that they wanted to invade and destroy Human colony worlds to use as their own. Since the Humans had only just recovered from the destruction of their original homeworld, their military was not up to scratch, and they could offer little resistance against the likes of the Klingons when they invaded (the Sontarans and the Quarians were usually beaten back by sticks and rocks, and if not that, their fleets were hit by asteroids and destroyed). Other races began to jump on the band wagon too, like the Reachites, the Krogans, the Volus and the Borg, and it looked as if 5 billion years of diplomatic supremecy was about to collapse around humankind.

The War
After reeling from the betrayal of some of their former allies, the Human race appealled to the League for help. League fleets were sent to reinforce the Humans and races like the Daleks, the Turians, the Elites and the Silurians rushed to defend Humankind. After buying the Humans enough time to get their forces ready, these races then aided the Humans in their fight against their political enemies in a conflict that lasted nearly three years before the League's sanctions finally dried up the aggressor's pool of resources. As the anti-Human army dissolved race by race, the Humans were finally able to properly re-establish their society.

Aftermath
Despite not being a particularly big war (compared to the wars of history) the war was dubbed the Third Galactic War, as it was the first war in League history to involve so many races with such a clear split of sides. It did have some benefits though, since alliances with the Humans between races like the Silurians, Elites and Turians were strengthened, and their inter-alliances were likewise strengthened. The war also allowed for an expansion of Humanity as their enemies (the aggressors) were heavily sanctioned by the League. After several local reformations and revolutions the anti-Human feel in the Milky Way Galaxy that had been stirred up by corrupt leaders soon diminished.