From the introduction:
Throughout history, mankind and the younger races have been manipulated and controlled by two ancient forces, the Vorlons, lords of order, structure, and discipline, and the Shadows, the bringers of chaos, competition, and natural selection. A war of ideology waged over millions of years, a war without end, came to an abrupt and sudden conclusion in 2261 when an alliance led by John Sheridan, Delenn, and the First One Lorien convinced the two self appointed guardians of the younger races to step aside and leave the galaxy - that now the younger races were ready to stand on their own. So it came to pass that the Shadows, the Vorlons, and all that was left of the First Ones left this galaxy for beyond the rim, and for the first time the young races of the galaxy were alone to find their own way between order and chaos.
While the First Ones are gone, however, many of their toys remain...