I can't remember if it was ever really talked about in the show, but what initially started the conflict between Centuari and Narns?
I know that the Narn used to be an agricultural people until the Centauri came along.
I also remember Londo talking about how the Centauri have a Manifest Destiny. Was it just a matter of the Centauri encountering the Native Americans of Narn and deciding to take over their land? Or was there something more?
The Centauri were an imperialist power, in their decline by the time they invaded Narn.
Once upon a time, the Centauri had been a powerful force in the galaxy. And they probably focused mainly on powers worthy of their notice while ignoring the "little fish."
Something between 100 and 200 years prior to B5, the Narn homeworld was invaded by the Centauri. The Centauri treated this as "liberation" and as "civilizing" them (think about the European colonial activities of the past centuries).
Narn were taken off-world to serve as servants and so forth, and turned into a "cheap labor force" to help support the Centauri "Republic" (which was never a republic at all... Republics don't have Emperors!). It's easy to do when the people you're subjugating aren't "really human" like you are (as our own history shows!)
The Narns were a pre-industrial, agrarian society. Probably close to what we were like from the 14th to the 17th centuries? They originally welcomed the Centauri (heck, the Centauri would've been accepted as almost being GODS at first!), but eventually (after a generation or two) I'm sure that the luster was lost. Technology doesn't impress if you've grown up seeing it your whole life, does it?
The discontent increased... and I'm sure that the repression increased accordingly. Who knows, the original "visitation" may have literally had good motives... but after 100 years of occupation, it had become a clear situation of MASTERS and SLAVES.
The Centauri, still watching their influence decline throughout the galaxy, were loath to let go of ANYTHING that they had their hands on... and jealously so, I'm sure.
So it eventually led to a civil insurrection, and undoubtedly a fairly violent one. The Narns "forced" the Centauri off their world, remember... and while the Centauri could've just bombed it back into the stone age (something that they actually DID do, though much later!), they chose to cut their losses and retreat.
Of course, the Narns had been doing all the "heavy lifting" of that society for a long time and had become technologically knowledgeable as a result. And I'm sure that the Centauri left not only their knowledge and some facilities, but probably a fair amount of technology (though not the really sensitive stuff) behind.
So, the Narns hate their prior "masters" and the Centauri resent their prior "servants" (and all the moreso because that was the last hint of the prior glory of a vast, conquering interstellar empire once held by the Centauri).
That pretty much explains everything...