Yeah, the Garmak piece was one of the better BG parts AoG did. An empire about the same size as the Centauri of that day (1464 - in the Really early days of the Centauri republic), with similar if not slightly higher tech (they were said to be slightly older).
The Centauri, having learned from their history with the Xon, expected conflict with the Garmak empire to be inevitable, and started preparing for a loong, hard war (and I suppose many centauri back then feared they might be the ones who would loose, as the Garmak were just as aggressive, and had a bit more of everything, having a "head start" into space).
But then the Garmak erred, erred badly - they thought the Minbari's policy of isolation a sign weakness, thought their age an indication of a civilization in decline, thought them easy prey for their forces, and the Minbari holdings an nice addition to their empire. So they decided to test their mettle with a few attacks on outer worlds - if successful, they'd invade, if unsuccessful, they'd apologize and denounce those atackers as pirates.
The attacks were wildly successful from the Garmaks point of view, taking those worlds without any real resistance - making them think they had been right in their estimation of the Minbari, that they really Were "paper tigers" and far beyond the time when they knew how to bite - so the Garmak sent in their forces.
On the Minbari side there was outrage, as those outposts had been undefended religous retreats. The council voted, the warrior caste mobilized (glad they finally had a war to fight - hey, they were bored...), and they then completely crushed the Garmak. The Garmak invasion fleets were met by superior numbers of ships armed with technologies they couldn't even dream of, and fell to the minbari advance. The minbari cut through garmak space like a laser through butter, savagely destroying every colony and fleet they found in name of their holy war, all but wiping them out (they stopped before toasting the Garmak homeworld, but that was it).
Now the Centauri were standing on the sidelines and saw how the enemy that had them really, really worried was dispatched like nothing by the minbari. What's more, after the deed the Minbari went back to their territory - something the Centauri just couldn't understand (why would an victorious force not follow up their victory?) But it was clear to them the Minbari had no interest in conquest (after all, it was obvious that if they had, they could have conquered Both the Centauri and Garmak simultanously loong ago), and thus would be likely to leave them alone as long as they left 'em alone too - something the Centauri could work with.
So they left the Minbari alone, and snatched up the now rather undefended Garmak territory. But they never forgot what they witnessed, and made every effort not to annoy the Minbari. Oh, there may have been points of dispute now and then - but every time the Minbari showed real interest, the cenaturi backed down quickly (especially since when it came to territorial disputes the Minbari had this quirk of sending Really impressive fleets as "show of force", so daunting that usually the Centauri forces retreated without even daring to protest - burning out half their fleet's thrusters with the speed of their flight :wink: ).