Cold War
Several Million years in the past, the Old Ones left the Galaxy to move bejond the Rim, only the Shadows and the Vorlons were left behind.
Or so everyone thought. Another reace of beings, the Ancients, also remained in the Galaxy to watch over the Shadows and the Vorlons and their dispute about managing the younger races.
While the Shadows seed war and strife and the Vorlons used iron discipline and order to prove their way of thinking right, the Ancients didn neither and so they were viewes as ‘being just like the younger races’ by Shadows and Vorlons alike.
When the Ancients formed an Alliance of four alien races and helped them in a way of a good parent, both Shadows and Vorlons allied for the first and only time in history, fighting against the Alliance to destroy every single bit of the Ancients and the races under their wing.
They succeeded in removing the Ancients and the other races, even stopping the desperate attempt to move bejond the Rim, but something unexpected happened, four of the massive number of vessels trying to move bejond the rim were heavily damaged and crashed on the single moon of the third planet of a yellow dwarf star.
Later this planet was called Earth.
Earth grew up with a large scar on the moon.
This is the story how mankind developed.