All There in the Manual: Most of the plot is derived from the Babylon 5 Wars tabletop game manual Wars of the Centauri Republic.
The Alliance: The Abbai are trying to create what will become the League of Non Aligned Worlds of the series, and there's a mention of the Nashani and Medushaan, two races near the Centauri-Orieni border that created the Free Worlds Confederation as a defensive alliance meant to be too big of an enemy for both empires to conquer while the other is still around.
Attack Drone: The Orieni Hunter Killers (both the ubiquitous but out of production Shining Lights and the superior but rarer Shining Star) are this, as the Orieni are not so callous to put pilots on a fighter whose only job is to ram enemy ships.
Boring Yet Practical: The decisive factors of the war are not advanced technologies or valour, but concentration of forces, logistics and industrial output, with Orieni long-term strategy being centered on dividing the more numerous Centauri forces on multiple fronts and capturing industrial targets and Centauri long-term strategy being centered in keeping the Drazi out of the war and maintaining their enormous advantage in industrial output. Also, operations are slowed down by the fact only the Minbari have FTL communications and aren't sharing.
The Battlestar: There are more than a few variants, with the most prominent to show up being the Orieni Prophet and Paragon-class command ships (enormous vessels dedicated to lead fleets, equipped with both heavy weapons and large hangars), the Centauri Centaurum-class battleships (mainly dedicated to kick ass, but carrying some fighters for cover), and the Minbari Sharaal-class warcruiser (Sharlin variant for planetary bombardment. Can still take on a small fleet of ships from other races, and carries fighters to avoid annoying attacks from enemy fighters).
Belief Makes You Stupid: The Orieni Roaring Rampage of Revenge against the Drakh was partially motivated by religion. Their assault on the Drakh homeworld, not militarily necessary, not only cost them much of their force and at least one military leader, but it also cost them the war when the Centauri realized the Orieni had removed most of their forces from the frontline and did the obvious thing.
Bullying The Dragon: Where the Centauri emperor ordered to stay away from the Minbari, some Centauri Noble Houses raided merchant shipping in their protectorate, and the Orieni tried to skirt Minbari space to attack the Centauri from an unexpected direction. Both Noble Houses and the Orieni stopped when a small Minbari force interrupted a battle between Centauri and Orieni and kicked both sides' asses until they ran in terror.
"There are always rumblings amongst the Houses. We are Centauri after all. You might as well ask us to stop breathing as to stop plotting and scheming."
Church Militant: Invoked by the Orieni: their military, while free of preachers and Blessed among their ranks, is called the Hand of the Blessed, and one of their chief duties is to protect the priests and the Blessed and to insure they can preach to subjects that don't worship the Living Gods yet.
Colony Drop: Do you remember the mass drivers, those weapons the Centauri used to throw asteroids at Narn in the series? The Centauri Noble House Refa (the same who bombed Narn) invented them in this war. It is mentioned that one of the first shots fired in anger ever changed the coastline of the target, with the implication that those used in the series were being fired at reduced power. They tried to do it with an Orieni command ship, but the target realized what was about to happen and its panicked reaction managed to cripple the ship with the mass driver.
Combat Pragmatist: Both sides use every trick they can, but the competent Centauri admirals are the best at it, making the Orieni believe they'll be able to attack an unarmed troop convoy who is then revealed to be a Centauri battlefleet in numerical superiority, luring the Orieni in areas where moving fast results in hull damage (how they first destroyed a command ship), and using and abusing the superior numbers of jump engines in their fleet for increased mobility and troll an entire garrison into not sleeping for days before attacking.
Continuity Nod: The story alludes to many parts of the future show continuity.
The Abbai are trying to form an alliance among the lesser powers to resist whoever will win the war.
Both powers are known to have a similar technological level to Earth Alliance during the series, and when pissed off Minbari show up to give them a lesson they complain that they could defeat them without their stealth.
Courier: As only the Minbari have full FTL communications (other races can only build and follow hyperspace beacons and receive the data from probes tapping in the beacon), most communication is handled through couriers (the Rumour-class, a faster and mostly unarmed version of the Strela-class light jumpship for the Centauri, and the Providence-class, a Steadfast-class corvette sacrificing the heavy weapons and most thrust for a jump engine, for the Orieni). Said couriers have diplomatic immunity at the start of the war, but after the Orieni shoot down two Rumours in the heat of a battle the immunity is off.
A more traditional courier is the Jade Hawk, a Centauri Ventrus-class light cruiser stationed at Usuuth that has to break through the Orieni invasion to warn the Centauri Republic of the enemy having violated Usuuthir neutrality and coming from the undefended side. As the Jade Hawk is mistaken for a Kendari-class fleet scout (and the Ventrus-class is in fact based on the Kendari hull, sacrificing its powerful sensors for a decent weapon suite), the Orieni dedicate too little ships to the task, and the courier actually overpowers and destroy its pursuers before escaping to hyperspace.
Crouching Moron, Hidden Badass: The Centauri were seen as too soft and pleasure-loving to fight an all-out war, and the Orieni counted on it. They were really surprised when the Centauri proved they not only had the stomach to fight the war but even escalated it, and had the appropriate reaction when they realized the Centauri had invented a cheap and fast way to pull a Colony Drop.
Curbstomp Battle: A few Centauri House Militias and the stupider Royal Navy commanders acted idiotically enough to allow the Orieni to crack them open without much a fight. Also, the Drazi tend to be at the receiving end of this every time the Centauri get their act together, and the Minbari are all too happy to dish them out on everyone picking a fightwith them.
Aside for when the Minbari show up, the Fall of Usuuth and the First Battle of Tagarhd stand out. The first saw the Usuuth coalition throw everything they had at the Orieni invaders, even scoring the first destruction of one of their immense command ships in the war, but had their entire fleet destroyed in less than a day (that mothership was destroyed by precisely the last Usuuth warship ramming it for mutual destruction) and their army surrender after a week. The second, a disaster so big the emperor managed to finally subordinate the militias to the Royal Navy, was described this way in the source material:
A House force built around House Varia resolved to fight to defend Tagarhd, the most valuable system in the House Varia sphere of influence. The House forces present were totally outclassed by the Orieni forces, but on top of that, they managed to set a new standard for inept performance. Debriefing of the survivors showed an almost total lack of control of the Centauri forces. They refused to accept the authority of Admiral Eddo Varia, ignoring targeting priorities, movement instructions and anything else desired, they refused to support each other, and even had they perfectly obeyed the admiral, his decisions were poor in any event.
Didn't See That Coming: Both sides were completely caught by surprise when the Minbari showed up from nowhere and started slaughtering everything in the area. The Centauri were surprised because these ones didn't know that some of their privateers were preying on their merchant shipping, while the Orieni had not knowingly violated Minbari space and considered the Minbari natural allies.
Didn't Think This Through: At least one Orieni archbishop of an occupied Centauri world had the Centauri temples prophanated and the priesthood executed. The military commander called him out on the stupidity of this act, as it explained the surge of resistance from the local Centauri. Note that the Centauri themselves don't do this, as they don't care of their subjects' religions.
Also, some Centauri privateers decided to prey on Minbari merchant shipping. As soon as the Minbari realized the Centauri Republic was just as surprised as they were at this idiotic act, the raiders were doomed.
Topping this, some Noble Houses suggested to have the first strike of the war bypass the Orieni border systems by passing through the Vorlon enclave. The Royal Navy, who had thought this through (and stopped sending and losing expeditions there when the Vorlon, after the disappearance of the last one, suggested to stop sending ships to avoid further 'accidents'), refused to even consider it, with the smarter House Lords being very glad that Vorlon space was forbidden by Imperial decree.
Some Orieni want to 'liberate' the Minbari Protectorate (the homeworlds of a number of aliens races controlled by the Minbari, either because they did need protection or, in one case, because they were stupid enough to attack at first contact and smart enough to surrender when they saw who they had pissed off). The government has thought this through, and isn't willing for three reasons: the Minbari serve the Living Gods (AKA the Vorlon), and attacking them would be sinful and treasonous; some of the races in the Protectorate needed Minbari protection because they had been nearly annihilated in the last Shadow War and still have to recover, and endangering them would be sinful and treasonous; attacking the Minbari would be suicidal.
Diplomatic Impunity: Some Centauri Houses believed the Abbai ambassador was using this to bring the Orieni informations on the defences of Centauri Prime when she decided to return home upon finding out of the war (she had simply decided to stay out of the way of the war, and had no intention to give the Orieni any information that she didn't even try to find out in first place), and sent privateers after her ship.
Dying Moment of Awesome: Ragnos Hessius' death. Former patriarch of House Hessius and 162 years old, he had long abdicated in favor of his son when he was diagnosed Keldarth Syndrome, so, knowing he only had a few months to live, pulled a Colony Drop on an Orieni command ship by blowing up the asteroid base he was on when the Orieni came too near.
Easy Logistics: Averted: the Centauri ability to both support the war against the Orieni and a fleet strong enough to crush the annoying Drazi raiders is shown as a decisive advantage since the start, and the greater logistical needs caused by the Hunter-Killers tend to slow down the Orieni rather often.
Elite Military: The Hand of the Blessed is noted to have outstanding training, with the Strike Force of the fleet and the Elite of the army accepting only the best candidates and then weed them out with training. It bites them back in the ass: as war goes on, the time-consuming training needed to reach that level of ability leave the critically short in terms of personnel, and their refusal to lower their standards means they don't have crews for their ships or enough people to fight the Centauri ground forces.
The Empire: The Orieni Emperor has no real power beyond christening new ships, but they're still an expansionistic empire bent on conquering everyone and submitting them to the Living Gods. The Centauri aren't much better: they may respect the culture of their subjects, but they also tend to enslave them.
Foregone Conclusion: Anyone who has seen the series know that the Centauri will win at high cost, and that the Abbai will eventually succeed in forming an alliance among the smaller races.
On the Centauri side, after attrition has killed all the idiots, we have: Landis Syma (doubly so because he not only was from the House Syma Militia but managed to get himself obeyed by a force from multiple Houses without being from the Royal Navy); Nattus Dannan, commander of the Thirty-Sixth fleet; Urza Ritan of the Third Noble Fleet (who House Forces won't disobey because being captured by the Orieni would be more merciful than his wrath. Got promoted to Grand Admiral for the Orieni asses he kicked);
The Fundamentalist: Both Centauri and Orieni are 'soft' version of this: both see their respective religions as the right one and consider everyone else stupid or misguided for not venerating the Great Maker (Centauri) or the Living Gods (Vorlon), but the Centauri don't care of other people's religion as long as they don't try to impose them on the Centauri themselves, and the Orieni won't actively force you to convert, only preach and bring peace and order. Of course there are some more straight version, like a certain Orieni archbishop who had the Centauri priests of an occupied planet executed (incidentally, there had been a surge into military resistance from the civilians).
Godzilla Threshold: The Centauri garrison of Bukaas, fearing to be hit by the Orieni with overwhelming force, decided to use their last resort to keep the local shipyards safe: severing the hyperspace beacon. Not only that put at risk of being lost forever dozens of ships from both sides, but there's also a risk of hyperspace shifting and nobody finding them ever again (already happened to the Woon homeworld, that did this to keep the Drazi from conquering them).
Hegemonic Empire: The Orieni Empire has shades of this, as the government of the member worlds is left to the locals, who are very happy of being Orieni citizens and only complain that their races lack native Blessed (telepaths) to serve as their governor and representative in the Council of Hierophants. The only reason they aren't fully one is why they lack their own telepaths: the Orieni killed them all and altered historical records to make it appear they never had them in first place.
Home by Christmas: Both sides (and especially the Centauri Houses) expect a short, victorious war, ending with the conquest of Orien after one or two serious battles (Centauri expectations) or the crippling of the Centauri Republic with the absorption of part of it (Orieni expectations). Of course they're wrong, and the Genre Savvy Surizeen, Minbari ambassador to the Orieni Empire, has only one thing to reply when told it will be a short war:
Hyperspeed Ambush: As in the original series, a squadron of warships emerging from hyperspace on top of the enemy fleet is a common tactic. The other variant, a group of warships waiting in ambush at a jumpgate, is less frequent but still present, especially done by the Centauri as the Orieni have an inferior number of jump-capable ships.
La Résistance: Centauri population on occupied worlds never stopped killing isolated Orieni or blowing up their tanks in what the Orieni called acts of terrorism. Also, the Usuuthir try desperately to force both sides away from their homeworld, but are completely outnumbered and outgunned.
The Revolution Will Not Be Civilized: Correntz was the seat of the Noble House worshipping the Thirdspace Aliens, and was occupied. The local resistance is much more violent than anywhere else.
Modern Major General: Most of the commanders of the Centauri House militias (sometimes led personally by the House lords) and a part of the Royal Navy admirals aren't exactly competent combatants, but are magnificent at politics. Luckily, attrition killed them off very quickly.
Nothing Is Scarier: After the Minbari showed their displeasure at the violation of their space, the Centauri raiders sent out six squadrons. Two have returned. Four have disappeared.
Oh Crap: Every single time the Minbari shows up, everyone else has this reaction fearing they've come to fight. One time, they did.
Only Sane Man: Emperor Jarah Rafani and the more competent admirals of the Royal Navy are this for the Centauri.
Orbital Bombardment: Features during every invasion of a planet. It was initially limited to conventional weapons (lasers for the Centauri, gauss cannons and sometimes nukes for the Orieni), but then the Centauri invented the mass driver...
Path of Inspiration: The Orieni religion has realized that the Vorlonvisited their ancestors and introduced telepaths among them and many other races, and worship them as the Living Gods, and consider a mission to submit all sentient races and make them worship the Vorlon too, even if they have to kill many of them to educate the rest. Also, at least according to the Centauri, the Orieni exterminates telepaths among their subject races (some Orieni have noticed the lack of telepaths among their subject races, and are either genuinely puzzled or take it as a sign that they're the master race).
Photoprotoneutron Torpedo: The Centauri make use of particle accelerators and light particle beams as light starship weapons.
Plasma Cannon: Heavy weapon of Centauri warships, and also equipping their fighters and ground forces.
Point Defenseless: Averted: Orieni point defense starts good and is increased twice (first in number and then in rate of fire of their gatling railguns), and the Centauri, once they realize the danger of the Hunter Killers, are quick to replace the insufficient numbers of particle projectors with greater numbers of faster-firing light particle beams.
Proud Warrior Race: The Drazi, who values strength above everything and, once beaten up in single combat, will accept orders by their defeaters. They also lack discipline and are grossly outgunned by the Centauri, with obvious results every time the Drazi pick a fight or the Centauri get annoyed enough to raid back.
Punch Clock Villain: How the Centauri treated them after the initial battles: they hired them. Often the survivors of the previous battles.
The Rogolon are another Proud Warrior Race. Being squeezed between the Centauri and Orieni borders, they decided to try and stay neutral. Emphasis on try.
Honor Before Reason: Their mentality on battle is to go and fight in single combat. In spaceship terms, this translates into small numbers of Rogolon warships going out, challenging the enemy fleet, and get ganged out when the enemy realizes that they're being serious.
Ramming Always Works: The Orieni make use of Hunter Killers, small fighters who attack enemy ships and installations by ramming them. They are also Attack Drones, as the Orieni, for all the terror caused in anyone who sees a squadron of these accelerating at ramming speed, are not so callous to waste pilot lives that way.
The Orieni have found themselves at the receiving end of this from time to time, usually at the hand of a crippled or otherwise doomed enemy ship (including but not limited to the last Usuuth warship ramming one of their command ships in a last act of defiance and, after this, an Usuuth merchant vessel ramming an Orieni one). Usually: during the First Battle of Kazat the completely outmatched Centauri garrison employed a squadron of shuttles crewed by volunteers in a desperate attempt to take down the flagship, and, while ultimately failing due the sheer size of the flagship, completely caught them with surprise (they believed the shuttles were carrying boarding parties) and filled them with justified terror and paranoia.
Reasonable Authority Figure: The Centauri emperor Jarah Rafani is this, and would have avoided altogether the war had the Orieni been more peaceful. As the Orieni pulled their trick with the Drazi, he's now trying to get the House militias to work decently.
Roaring Rampage of Revenge: Two of them. The first is the Royal Navy reaction to House Syma's attempted coup, consisting in destroying any Syma ship or base until the entire House leadership was dead. The other was the Orieni reaction to the Drakh perceived betrayal and their identity as Shadow servants, and was a genocidal attack on their homeworld.
Shown Their Work: Both the B5 Wars fluff and the author show they know more than a little about the importance of logistics and industrial production in warfare, with the Centauri superior industrial production being a decisive factor in the planning of both sides and in the outcome of the war.
A Simple Plan: The initial Centauri plan is to smash through Djost's defences and attack Orien, and fails due the sheer might of Djost's defences, the incompetence of the Centauri House Militia commanders (and of some Royal Navy admirals), and general bickering, with the Orieni losing sleep in the search of whatever sucker punch had been hidden by the first failed assault (it took them a while to realize that yes, the first assault force could really was that incompetent). The Orieni initial plan is to let the Centauri smash themselves against Djost's defences before bypassing the Centauri border defences through neutral Usuuth space and hitting them hard enough to force them to surrender and give up the border area, but it fails due inadequate logistics, the sheer numbers of the Centauri, running in some actually competent Centauri admirals, and their Drazi allies proving themselves too incompetent to distract any meaningful segment of the Royal Navy (that in fact took ships from the Drazi border to reinforce the frontlines).
Space Fighter: A lot: the Drazi use the old Dragon and the more recent Adder-class fighters (both outmatched by the Centauri, but the latter in lesser measure), the Centauri have the Glaive light fighter for interception and dogfights and the Phalan assault fighter to attack enemy ships with either a bigger plasma weapon or missiles depending on the version, and the Orieni have the Templar interceptor for combat against other fighters and covering the Hunter-Kllers. All of them are outmatched by the Minbari-made Tishat (the predecessor of the Nial), who has superior speed, greater manouverability, ludicrous firepower (the fighter-mounted fusion cannon outguns the anti-ship plasma gun of the Phalan and is faster-firing. The Tishat has two of them), and that annoying stealth ability that prevents enemy weapons from locking on it.
Space Pirates: There's many of them, mainly Drazi seeking glory and minor sons of Centauri houses trying to amass enough wealth to become important. They also have the strange habit of biting more than they can chew, like Centauri Q-ships or Minbari shipping.
Prequel: This is what made the Dilgar invasion even possible: not only the mass driver so loved by the Dilgar was invented by the Centauri in this war, but the crippling of the Centauri Republic in the war against the Orieni and the latters having a resurgence around the time of the Dilgar War are the only reasons the Centauri didn't just invade Omelos as soon as one third of the Dilgar fleet was tied up on the Drazi front and the rest busy invading the rest of the League.
Sufficiently Advanced Aliens: The Vorlon, big time. They actually showed up in person only in a flashback, but the Orieni worship them as their Living Gods. If you ask the Centauri and the Minbari, however, they will point out they're this trope.
Also, the Thirdspace Aliens: they have technology comparable to the Vorlon, telepathic powers to subdue everyone unlucky enough to meet them (Vorlon included, if they don't shoot fast enough), and a Religion of Evil dedicated to bring them in our universe to raze it.
Too Dumb to Live: We have two magnificent examples: the first is a group of Centauri raiders preying on Minbari merchant shipping after the Minbari warned the Centauri and the Orieni to stay away from their space for the duration of the war (the Minbari were mildly irritated and massacred the first Centauri force they could find before repeating their warning, adding that next time they wouldn't be so merciful); the other is House Syma trying to take over the Republic upon the death of emperor Jarah Rafani in spite of being during a war and an heir being available (the Royal Navy did not look the other way but decided to make an example of House Syma).
About those raiders... Some ran from their base in terror after the Minbari expressed their displeasure and returned to the Republic, but others stayed and continued raiding the Minbari, even after four out of the first six squadrons sent out to raid disappeared. Astonishingly, some remained even after that, and when the Minbari tracked them they attacked. We don't know if the Minbari laughed histerically before wiping them out.
The Theocracy: The Orieni Empire. There is an emperor, but its most important duty is to name new warships, while all the power resides in the Council of Hierophants, the logic being that its members are all blessed by the Living Gods with the gift of telepathy and, being elected by all worlds of the empire, represent every single population. By what the Centauri can bring evidence it's not a coincidence, the Blessed just happens to be all ethnic Orieni (that is, every single Blessed comes from the Orieni race, as opposed to be an Orieni citizen of Phanop or Choaka race, to cite two of their subject races).
Touched by Vorlons: Telepaths are telepaths because the actual Vorlon have manipulated their genes or those of their ancestors. The Orieni constantly point it out by calling their telepaths Blessed.
Unobtainium: Quantium-40, as in the series. A plot point is that the Centauri have a lot more Quantium-40 mines (and a proportionally larger production of jump engines) than the Orieni, giving them a crucial advantage in strategic mobility thanks to the larger number of jump engines in their fleet. You may understand the horror of an Orieni officer when he found out that, following his orders, he refused the surrender of an unknown Quantium-40 mine and destroyed it from orbit (with gauss cannons, and not with nukes, thankfully. Meaning that the vein was not disintegrated and the mine could be rebuilt).
Utopia Justifies the Means: The Orieni wish to bring the entire galaxy into order and worship of their Living Gods (the Vorlon, who, if asked, will admit they are not gods), so that every race will prosper. To do so they attacked and conquered many races, manipulated the Drazi into attacking the Centauri (who, upon discovering a Drazi ship with Orieni-exclusive weapons, decided they had enough of their meddling and declared war), and committed various attacks on civilians (this act openly disgusted every Orieni who did it, and they try to not do it as long as they can). Also, the Centauri are convinced they exterminated telepaths in their subject races, so that their empire will be ruled only by 'Blessed' of ethnic Orieni origin... And the Centauri Telepath Guild knows they're killing any Centauri telepath they capture.
War Is Hell: Entire planets have been razed, and the population of Usuuthir (a neutral world that the Orieni invaded to attack the Centauri from an unprotected side) was reduced from a few billions to less than a million by the repeteated battles for its control and the first ever mass driver strikes, fired to destroy Orieni troops left there from a previous occupation and dissuade the natives from resisting Centauri occupation.
We Are Struggling Together: Centauri infighting is so great and infamous that it facilitated the initial Orieni defence of Djost beyond their wildest dreams (to the point that the Orieni believed the utter incompetence of the initial attack force was a cover for something bad, and it took them a while to realize that it was true idiocy at work), and House Syma actually launched a coup to try and seize the throne when the emperor died of old age. The last finally exhausted what patience the Royal Navy still had, and their Roaring Rampage of Revenge put an end to this through sheer terror of a repeat performance.
We Have Reserves: How the Centauri resisted the initial Orieni offensive: they had enormous numerical and industrial superiority.