Originally, the Centauri Republic
conquered the Narn
at the beginning of the 21st century. (One Hundred Years before Babylon 5
. The Narn
welcomed the Centauri
with open arms and the Centauri
occupied them for a hundred and thirty years and strip mined the planet. The Narn
were able to defeat the Centauri and remove them from their homeworld
by learning their technologies and turning them against them. Though the Narn
managed to remove them, through a war of attrition
. The Centauri
had turned Narn
from a forest planet into a bleak and barren one.
This is called the Centauri-Narn War
After this Narn Victory, it is conceivable that the Narn formed their interplanetary government: The Narn Regime.
After a thousand years 'asleep' an ancient race millions of years old called, "The Shadows", awoke and offered their help to the Centauri. Using this help, in 2259, the Centauri attacked various Narn Outposts, causing the Narn-Centauri War. The war was poorly fought, by the Narn, who were severely outmatched by the superior Shadow technology, specifically, the Shadow Mothership.
The war concluded when Cantauri forces invaded Narn Space and using Mass Drivers to bombard the surface of the Narn Homeworld with asteroids (a weapon which is banned by almost every civilized race).
This attack on the planet had the devastating effect of destroying the whole Narn infrastructure and pushing rocks and dust into the atmosphere, it will take about a hundred years for the dust to settle. After this attack, the Narns unconditionally surrendered. And the Narn Regime is dissolved.
The Narn-Centauri-War began in 2259 in the episode "The Coming of Shadows" with the Shadow/Centauri attack on the Narn Colony in Quadrant 14.
The War concludes in The Long, Twilight Struggle when the Narn Homeworld is bombarded by mass drivers.
The Narn are enslaved for over a year and a half until Londo Mollari orders the Centauri Republic to withdraw from their Narn Occupation.