Armageddon Steel Legion
Take the Death Korps of Krieg, replace the doom and gloom with slightly less doom and gloom, throw them into the largest land war in modern Imperial history, and give each man his own Chimera BEEEIIHHHNNBLLAAAAIIDDDEE. What you get is the Armageddon Steel Legion, one of the most heavily mechanized Imperial Guard units in the galaxy, with a penchant for fighting Orks.
The First War for Armageddon
So, minor bit of history. Armageddon is pretty much a big ass factory, that on three occasions has been trashed. The first time, Angron decided it would make for a nice summer home, and decided to party for a while with a couple thousand Bloodletters and fifty thousand Khorne Berserkers of the World Eaters. Much partying was to be had before someone called the cops with a noise complaint, and the Grey Knights, Space Wolves and whoever hadn't been killed managed to take back the planet, with ninety Grey Knights dying to banish Angron.
When the party was over, the Inquisition decided to take out the trash, meaning that they forced the entire population of Armageddon into labor camps and worked them to death on suspicion of HERESY, before repopulating the planet. This pissed off Logan Grimnar and the Space Wolves to no end, because they had the audacity to think that actually helping people was the right course of action. You know whats really stupid? The Imperial Guard fights Chaos constantly, successfully, and kept from turning (most of the time) because of their faith that the Emperor would protect them. Which means, of course, that if knowledge of Chaos and the Grey Knights spread it would actually improve morale ('cus hey look, space marines wtfpwning daemons, no need to panic, the Emperor Protects). Idiots.
Armageddon 2: Electric Boogaloo
Jump ahead a few centuries, and we get the real origins of the Steel Legion. Warboss Ghazghkull Mag Uruk Thraka thought that Armageddon would be a nice place for a WAAAGH and so there was a big ass fight. Commissar Yarrick led the defense of Hades Hive, holding out for months without support against a gigantic Ork offensive. This is where Yarrick first earned Ghazgull's respect; countering evey attack plan the warboss had, and sending crackheads on suicidal charges into ork camps with explosives strapped to them.
With the Guardsmen stretched thin, the Blood Angels arrived to pull their asses out of the fire, doing what Indrick Boreale couldn't and performing a successful Steel Rain attack. The entire company dropped into the Ork Horde and fought nonstop until the Orks fled. Ghazghkull said he'd be back, so the Guardsmen began prepping for war.
Having lost the majority of its PDF and most of the off-world regiments, all that was left was charred corpses and scorched metal. As the spess mahreen apothecaries solemnly collected their fallen brothers' gene-seed, thousands of medicae could be seen retrieving countless standard issue adamantium balls, as well as the odd pair of power balls. Manly tears were shed on both sides and the repatriation of the planets defences would begin.
The Third War for Armageddon
Preparing for his return, the Steel Legion became insanely fortified, with the Munitorium granting them a lessened tithe to keep local forces numerous enough to defend the planet. Millions of tanks, transports, and any other vehicle they could build were used in a gigantic buildup for the Orks return.
When they finally did, the Third War quickly escalated into the largest Ork WAAAAGH in history, drawing in dozens of Space Marine chapters, Imperial Guard regiments and even Titan Legions. After a mostly successful initial defense, the Steel Legion was slowly pushed back into heavy street fighting, turning the war from the Africa Campaign into Stalingrad, with tanks rolling off the factory floor into combat zones. The war mostly stopped, when Armageddon entered its "Season of Fire" (a major period of volcanic activity), so while the Orks managed to successfully capture almost half of hive-cities, the cities in which they were defeated became unreachable. Orks got bored, and withdrew most of their forces to space, though still a lot of them remained on Armageddon and continued to fight. Both sides declared this as their victory. Steel Legion units now use the permanent Ork infestation for training purposes, keeping them away from the remaining major hives.
Steel Legion Troops
What can you say? They're German Mechanized Infantry- (TECHNICALLY,they appear to be German Fallschirmjägers in British 1914 longcoats without parachutes, and with tanks.) IN SPAAAAACE! They wear bitchin' trench coats and gas masks, they fight Orks on a regular basis, and they have access to a metric fuckton of armored vehicles. They probably use Chimeras as condo(m)s.