Mordian Iron Guard
"Discipline is the soul of an army. It makes small numbers formidable; procures success to the weak, and esteem to all."
- – George Washington
The Mordian Iron Guard are an army of the Imperial Guard, hailing from the planet of Mordian. They are renowned throughout the Imperium for being right hard bastards, never backing down and prepared to die to the last man as necessary, as well as being from a planet with an adjective for a name (the Earth equivalent would be if Italy were named Italian).
They are also renowned for wearing spiffy dress uniforms, although they're not as classy as their equivalents in the Praetorian Guard.
Although uniform colors likely vary between regiments as with most worlds (even Krieg).
Their uniforms are some of the best, only surpassed by the Terrax Guard. Fancy enough to show the glory of the Imperium and Pride of the Imperial Guard but not so fancy it makes you a target a blind Ork could hit from half a kilometer away *cough* Vostroyan *cough*.
There are Imperial Guard regiments that rely on using well-planned strategies and combined arms tactics to overcome their foes. Then there are those that don't rely so much on strategy so much as throwing hordes of guardsmen into the fray, although said guardsmen are often some of the most badass motherfuckers in the entire galaxy. Then there are the Mordians.
The Mordian Iron Guard technically belong in the second category, but are so far around the curve that they start veering back into the first category. The first thing to consider is that they hail from Mordian, a world that actually has two classifications; half Death World, half Hive World.
Normally these two types of worlds are mutually exclusive, since a planet where everything is trying to kill you would put a lot of people off the concept of actually erecting massive towering cities on it. Not so for the Mordians. The planet is tidally-locked (i.e. it spins at the same speed it orbits its sun), so one side of the planet is constantly bleach-roasted by the Mordian sun, and the other side lives in zero degree dick-freezing perpetual night. Additionally, the absence of strong winds on the planet creates a thick and oppressively dark atmosphere (which makes no sense, because temperature differentials that large should cause huge winds as the hot air rises and the cold air sinks -- living in the twilight zone would be like living in a permanent hurricane). The Hive Cities are built on the night part.
Thus, every Mordian native is also a hiver, with years of experience ducking and killing inside narrow corridors where hive gangs exist on every level, and crime is rampant in the places where the sun-don't-shine (i.e. that entire side of the planet). Unsurprisingly, this has earned Mordian the dubious honor of having the highest suicide rate in the Imperium after the destruction of Nostramo. It got so bad that up until recently there was a notorious chaos uprising and the Mordians had to fight tooth and nail just to keep their semi-hospitable half of the planet from being overrun. Between that and the extremely strict rationing that leaves the planet on the verge of rebellion at all times, the Mordian Iron Guard has learned to practice iron hard discipline (hence their name) and constant regimental routine as part of their secondary role as military police. Nearly every citizen on Mordian has served in the planet's military, as it was the only way to maintain global discipline.
Consequently, Commissars find a tour on Mordian as rather boring, and a little intimidating as the Mordians themselves will penalize Commissars that falter in their duties. Again, absorb this for a moment. In almost every other part of the Imperium, Commissar shoots you for lack of discipline or cowardice. On Mordian, You shoot the Commissar for lack of discipline or cowardice. This is then reflected in their form of battle tactics. Rather than ride Chimeras or wear camouflage, Mordian guardsmen march into battle in brightly-coloured cloth uniforms and bearing equally flashy banners. They then form up into neat firing lines or squares and shoot at their enemies in disciplined volleys. They don't break formation, they don't take cover and they never, ever retreat. While this is incredibly ballsy even by Imperial Guard balls-of-steel standards, it is also incredibly stupid because on the battlefields of the 41st millennium there are oh-so-many ways to wipe out massed formations of infantry out in the open. These sorts of tactics were on their way out in the American Civil War (appropriate as Mordians look quite a bit like US Army from that era) when caplock minie ball rifles and rifled muzzle loading cannons began to appear. Even Space Marines will take cover whenever they can. But, somehow, the Mordians still survive their battles and keep going. Considering the horrors the Imperial Guard fights, you have to wonder how their training conditioned them so well. Then again, maybe you don't want to know.
Also, the Mordians make frequent use of sanctioned psykers, so that probably goes a long way to explaining their success. The gunmen pin the enemy, the psykers erase the enemy (quite literally). Their favored special weapon is the humble grenade launcher and their favored heavy weapon is the trusty lascannon.
Although a few sources note that this baffling lack of common sense causes their adversaries to underestimate them, it still doesn't really make up for employing infantry tactics that have long been redundant. That being said, the Mordians do have a few tricks up their sleeves, like flak and carapace armour actually sown into their rigid uniforms (which makes it both elegant AND protective), as well as having heavy armour companies with over a dozen Baneblades each. They also have 'veteran' companies who wear the incredibly-more-badass darker uniforms who do actually observe real infantry tactics and perform the scout work.
It's important to remember, however, that the fact the tabletop rules gives them advantages with two or more models (or tanks within three inches of each other) to Overwatch. So, it seems their parade line fighting is more likely how they fight on the defensive to repel enemy attacks than how they actually assault positions. Especially since their Veterans who actually do the initial attacking use proper tactics. On the defense, a gunline backed by tanks annihilate any attempt to push forward and any potshots are shrugged off by their mix of flak and carapace armor. Attempts to hunker down are wiped out by either lascannons, grenade launchers, or battle cannons, and the enemy has to endure constant strikes from Basilisks, Griffons, Wyverns, and infantry mortars coming in, after being pinned, and when falling back. On the attack when Veterans aren't enough, the Mordians most likely spread out and stagger their lines to defend against explosives but otherwise fight as advertised as a grinding advance to keep the enemy down so the Veterans and tanks and lascannons can do the rest. Cover is nice against artillery but when you're wearing 40k flak and/or carapace armor anything short of a direct hit from artillery isn't really a threat (lore-wise, not tabletop) and so you can march through waves of explosions without flinching. Ironically, it could be said the Mordians are more up-to-date with 40k era tactics while the Cadians and other such regiments are fighting the Eternal war with outdated tactics (fighting like it's a 21st century war even though their armor makes artillery far less dangerous than to a modern soldier).
However, one must note that the fact that they're never taking cover means that they're not spending any time not aiming or not firing, meaning that relative to other armies, they could keep you pinned a whole lot more easily (in fact instances in fluff and descriptions of their tactics seems to imply they took the "maneuver" out of "fire and maneuver" and just utterly suppress the enemy for lascannons and grenade launchers to finish off by blasting away the enemy and their cover together). So while they might die off really quickly, they'll do a hell of a lot before they drop. Since their armor has both flak and carapace armor woven into it,(not represented currently on tabletop) the Mordian Guardsmen are well protected against concussive force, shrapnel, and even against direct fire. So, not only is it a wall of lasers shooting at you, shooting the wall has about as much effect as spitting on a tank. Besides, they probably don't stand in a row, they probably have the first row kneel and the rest stand(leaving the men kneeling more accurate then the men standing as your lose a third stability point as you get closer to standing). Then they have four rows of lasguns firing in a concentrated barrage at the same target. Then remember how long and deep the gun line is, given how numerous the Guard is. That isn't even accounting for man-portable mortar teams, vehicle-mounted mortars, basilisks, manticores, and the armored companies of over a dozen baneblades each.
That's scary and is the perfect tactic for the Guard because their reserves have reserves. It also happens to be the perfect tactic against Tyranids and Orks. The Tyranids are simply that numerous and the Orks are specifically stated to miss anything they aim at, so bunching up into a single target is the best defense against them. Maybe good against Necrons, too, since cover really won't protect you against their weapons. Firepower will.
In the worst-case scenario in which you only have Mordian infantry going against a target due to usual Administratum competence, you'd still do very well. Your Veterans are Grenadiers with all the kit that implies and infiltrate (kinda like the German stormtroopers of the First World War) while your enemy is suppressed by the Mordian battleline. Mordian infantry use a lot of lascannons and grenade launchers, so the lascannons could easily kill enemy heavy weapons emplacements and light vehicles, Veterans could sneak close and enfilade the suppressed enemy and melta-surprise heavy vehicles, and as you get closer the grenade launchers would simply devastate everything. All while the enemy can't do much of anything to avoid their demise since popping out of cover to shoot back just gets them roflstomped by the thousands of extremely precise marksmen that are the Mordian infantry. Tactically, the enemy's best option in this situation is to fall back under the cover of artillery. You can only fallback so far before it counts as defeat no matter what you tell yourself. And then you're blown up by Basilisks.
The Mordian Iron Guard are well known for being extremely effective and successful and are among the absolute best soldiers in the entirety of the ridiculously insanely massive Imperial Guard. Here's hoping the Imperium adopts their methods (including the civilian military service and discipline), since it seems to deal with crime and cults on their own world with wondrous success and the soldiers are stupendously full of win. Imagine quadrillions of soldiers dressed like the US Marines' dress uniform with golden decoration on their shoulders marching into fire, over trenches, non-stop over every and any opposition. Orks, Necrons, Tyranids, and daemons fall before them and the last thing the monsters hear is the incessant beat of marching feet. (s/Say that to a Catachan, why don't ya??)
There is another reason why their tactics likely work: the main enemy of nearly every Guardsman throughout his life are either rebels or less-than-Imperial-tech aliens. Against both of those flak armor is basically invulnerable and the lasgun is basically God's own anti-sunnuvabitch-machine. Now combine that with the Iron Guard's tactics and the fact they have both flak and carapace armor woven into their uniforms and particularly effective lasguns (Triplex) and each squad has a Leman Russ firing over their heads and their regiments have super heavy COMPANIES. Yeah. Fuck yeah. It's entirely possible they use normal tactics when facing things that their normal formations don't work. Also, they use parade formations, but that doesn't mean they always form up as a battle square and march forward, there could easily be far more varied formations they use depending on the enemy and situation.
Fall of Mordian
The planet Mordian along with the entire Stygius Sector was invaded by Chaos Forces led by warbands loyal to Tzeentch. The Iron Guard held their ground as reinforcements arrive, but there were too many heretics and slowly the Imperial defenders are being bled dry. (Didn't help that the dead are being raised as zombies or that the Warp claimed several Imperial ships and an entire planet during the conflict.)
Further reinforcements came in as an Eldar Strikeforce from Craftworld Ulthwé aided the Imperium. Being the cryptic pricks they are, they tell the defenders that they can't win and advised them to leave the sector. (Seeing as the Chaos Invasion is led by the Thousand Sons under Magnus as well as M'kachen, they are kinda right). Even though the discipline of the Iron Guard kept them from being overrun, they agreed to retreat, and evacuated what they could. It probably stung, but you'll never see a Mordian getting mopey about it.
However, thanks to the perseverance of the Iron Hands and its successor chapters, Chaos forces were finally driven off Mordian and reclamation of surrounding worlds continued in order to secure the adjacent passage through the Great Rift. It's not quite clear if they reinforced the Mordians or joined a Mordian attempt to re-take Mordian or if they did it on their own. Seems likely that the planet was secured due to their arrival instead of evacuating the planet. Speaking of, that the Iron Hands did this proves the Eldar were wrong. Again.