After the defeat of the Shadows the Drakh, a servant race of theirs who handle things like keeping things ticking over while the shadows are in their thousand year hibernation, taking out the bins, feeding the fish etc. The Drakh used to have a settled world, but this was rendered uninhabitable by the Orieni during the Orieni-Centauri war about 250 years before the series. The Orieni were a powerful stellar empire that worked out the Vorlons were the gods they worshipped, and had a powerful telepath ruling class. This made them potentially even better lackeys for the Vorlons than the Minbari. They were expansionistic, as were the Centauri at this point and before long they were skirmishing and it became a shooting war.
The Drakh tried to manipulate both sides to extend the war and end the two major organised empires in that section of the galaxy. They backed a coup by a Centauri house against the Emperor, and tried to act behind the scenes with the Orieni as well. The Orieni worked out what was going on, and worked out where the Drakh were coming from, and mass drivered their planet to bits. The Drakh became a nomad race at that point, eventually resettling on Zahadum. The Centauri house they manipulated failed to bring off the coup, and was completely wiped out.
So at that point the Centauri know of the Drakh, but everyone who met one is dead, and the Orieni lose the war and their empire collapses.
This means at the time of the Shadow war there is not a small number of monolithic empires, some closely aligned with the Vorlons, but a large number of bickering races, a decayed Centauri Republic and an expanding Earth Alliance. The short term failure of the Drakh to place a puppet on the Centauri throne leads to a long term pay off in terms of the Shadows plans.
The Drakh, once the Shadows have gone, have lost their gods, and then their new home. They scatter, and some try to settle in Minbari space (and get Whitestared to bits) and some settle on Centauri prime. They also find an old shadow base of Xadam, and pick up an old and crappy version of the Shadow planet killer, which they try to use on Earth (A Call to Arms).
The Drakh on Centauri Prime use Keepers (telepathic symbiotes) to manipulate the Regent (the little Centauri chap who likes interior decorating and is completely harmless) to rebuild the Centauri military and put aside some Demos class ships for the Drakh to add organic control pods too. The Drakh plot to split the Interstellar Alliance by creating a war, and use these remote control ships to raid Alliance shipping, brutally massacring convoys rather than indulging in traditional piracy. They leave a trail of evidence to point to the Centauri that the IA eventually put together.
However the Drazi destroy some of the raiders ships, salvage the control pods and launch into a research project on them but do not divulge their existence to the alliance.
Everyone rounds on the Centauri, Londo denies everything (because he really believes they are being framed, and to be fair, they are). The Centauri leave the IA, and sanctions are imposed by the IA members on the Centauri.
This leads to skirmishes when the Centauri try to break a Drazi blockade, obliterating a Darkhawk squadron when they come up against a Demos squadron. Garibaldi fails to have the White Star fleet intervene and prevent conflict as he is drunk as a lemur.
The Drazi approach the Narn, and organise a pre-emptive strike on the Centauri homeworld. Sheridan learns of this too late to stop it, and G'Kar is on Centauri Prime trying to calm things down at the time.
The Regent orders the Centauri Home fleet away, turns off the planetary defence grid, and the Narn and Drazi arrive unopposed and bombard the planet. By this point Sheridan has discovered the involvement of Shadow tech and that the Centauri have been framed, and forces a truce. The Centauri pay reparations, the Regent takes the fall and Londo becomes Emperor. In return for not detonating the nuclear weapons hidden across Centauri Prime Londo accepts a Keeper, and from then on his actions are manipulated by the Drakh. Londo keeps Vir as far away as possible from him, knowing the prophecy that Vir will succeed him as Emperor.
The Drakh force Londo to promote an ambitious and a bit nuts guard captain, Durla, to a ministerial position. Durla is manipulated by the Drakh, but not through a keeper, as he ambitious and ruthless enough already. They manage to slip him advanced weapons tech through dream visions using telepathy, and he marries Londos third wife (the one who slept with G'Kar, tried to kill Londo, been disgraced, eventually become a spy for the new regime Durla is putting together, used Vir as a cover with Vir believing she loved him, been revealed as a fraud to Vir by Galen and then placed under a geas at Virs request so that she loves him desperately, and is then placed to spy on Durla as Vir sees his pivotal role in events).
Durla is key to the Drakh plans, transforming Centauri Prime from a decaying and decadent republic into an efficient if facistic society. His funding of archeological digs reveals to the Drakh the location of Xadam (which the Shadows seemingly abandoned given the relative low tech of the planet killer there, and given the ease with which the Vorlons would have destroyed it), and the Centauri military build up provides a great distraction to the IA. The Centauri pick off some small worlds on their borders that are not Alliance members or league members, gaining further territory and resources.
There is a suggestion the Centauri covertly fund raiders in this period, but it is never proved totally.
Londo eventually grows fey. He discovers he can put the Keeper to sleep by getting really drunk, and uses this to keep a complete and accurate record of events.
The Drakh, getting a bit over excited, attack Earth with their shiny new planetkiller, only to be foiled by Sheridan and the guy from Candyman, along with Dureena. The Drakh infect Earth with a nano-virus, and the Excalibur is donated to Earth force to search for the cure (Crusade). Earthgov continue their own shadow tech based plots, despite the massive help the Alliance and Minbari are giving them in trying to find a cure. A cure is eventually found and within about 5 minutes the humans are saying "Come on, what have the Alliance done for us recently?".
The Drakh take a beating with their attack on Earth, but fortunately they are compartmentalised enough that no individuals captured reveal the plan for the Centauri. Eventually with their military build up complete, the Centauri plan to pay back the Alliance for their attack, launching simultaneous surprise strikes on league homeworlds to start a new era of war and chaos.
However Londo, who has been using Vir as a cats paw to create a revolutionary force, grows old and a bit mad. However when the Drakh plan to lure Sheridan and Delenn to Centauri prime works, he engineers their escape and his own assassination by G'Kar. Unfortunately Virs revolution occurs almost simultaneously, revealing the Drakh prescence on Centauri Prime. The Drakh tell Londo to do something about it, he tells them where to shove it, and they reveal they are not bluffing by devastating a third of Centauri Prime with fusion weapons.
After Londos death, and the death of Durla, and most of the Centauri government, and open conflict between Virs revolutionaries and the Drakh, the Drakh abandon Centauri Prime. Vir, proclaimed Emperor, orders the military, who are under the command of someone whose loyalty was to the Emperor, rather than Durla or the remaining head of the great houses, to hunt down the Drakh, which proceeds and the Drakh are driven from Centauri space.
The Centauri and IA essentially have a 20 year cold war, a hot war averted by Londos 20 year plot he manages to keep secret from the Drakh by dropping subtle hints to the people he wants to carry it out that there is something going on and perhaps someone should do something about it, and by preventing their deaths at the hands of Durla or the Drakh on several occasions. Londo plans his own death as marking a break in the regime, taking the blame for the Drakh plots and providing a scapegoat for the various other problems, allowing a new order to emerge under Vir.
So Londo is a sort of hero, Vir is a more understandable sort of hero, and G'Kar kills his friend to save him, and Sheridan and Delenn.
Thats basically why. The Orieni stuff is from WCR, the rest from the series and Centauri trilogy. So most of this is canon, and it all hangs together quite well.