As the name suggests, the 1st Harknian is the first regiment raised from the planet Harknia. Since 742.M41 Harknia has raised multiple regiments for service in the imperial guard, the majority of which are mechanized regiments.
The planet of Harknia was liberated from the grasp of Ork Warlord Thurgg Megazappa around 400.M41 after being captured during Megazappa's Waaagh through the eastern fringe sectors close to Ultramar. Imperial fighting was centered around the systems closer to Ultramar, with heavy combat including space marines from the Ultramarines and Black Templars chapters. These worlds were locked in war for many decades, and once the main waagh fell apart and the ork forces attacks crumbled, other conflicts overtook the sectors need for re-conquest. Thus the planet of Harknia was not liberated for close to 300 years after it was taken from Imperial hands.
Now a planet that is equally agricultural and industrial, it has very little pollution and is thus a very pretty world, however before the Orks arrived it had been an imperial shrine world, with special ties to the Primarch Guilliman who had landed on it personally to liberate it during the Great Crusade. During the Ork occupation the world's holy ornaments were all destroyed and turned into slag to fuel the ork Waaagh, and since its re-colonization it has been re-classed as a agricultural world with industrial cities.
In early 741.M41, the population of Harknia reached a quantity that required it to prove a tithe of troops to the Imperial Guard. Harknia already produced copious amounts of parts for armored vehicles, mainly the chimera transport, and the planetary government decreed that their local regiments should be equipped with the APC's to offer them the best protection. Thus the 1st became a mechanized regiment, with a total of 15 Mechanized companies and 3 Armored companies.
Shortly before the regiment was commissioned as an active combat regiment, the Tau Empire invaded systems near to Harknia, and the Imperium responded by launching the Damocles gulf crusade. The regiment was commissioned and whipped straight off to fight the Tau menace. Upon its first deployment, the Regiment was counted at a total of 4618 men.
The first time the regiment saw action was during the invasion of Ka'syth, a dwarf planet held by the Tau on the outskirts of their empire. It was a strategic jumping off point for the Crusade forces, close enough to the core worlds of the Tau to allow quick warp jumps for re-supply, but not so close that it could become victim to surprise raids.
The Planet had little orbital defense, however, over the main city on the planetary surface an airborne spaceport structure hovered, holding multiple Barracuda Air Superiority fighters, which provided a large threat to landing imperial troops. As the only regiment of the invasion force with training in fast mechanized assaults, 4 companies of the 1st Harknian would deploy onto the structure to disable it and its compliment of aircraft an hour before the main imperial drop. They would have air cover provided by Thunderbolt fighters, and transport provided by multiple squadrons of Valkyries from the navy fleet. There were only enough Valkyries to land 4 companies simultaneously, but it was reasoned that the 1100 troops would be enough to clear the station. The rest of the regiment would deploy with normal equipment to the planetary surface with the main imperial assault force.
The 4 advanced companies managed to land and capture the structure in around 45 minutes, though the heavy fighting cost more than 300 casualties. The main force then Continued onto the surface, and with minimal casualties to the landing parties managed to capture the principal city. The 1st was withdrawn from the action towards the end of the fighting to join the main crusade towards the Dal'yth system.
The 1st spend the rest of the crusade fighting on the planet Dal'yth with the main crusade force, where they quickly learned of the skill and courage of their foe. The Tau had a remarkable ability to mark and destroy armored vehicles and columns before they were spotted and slinking away unseen, leading Colonel Hargrieve to withdraw most of the Chimeras from the front line when not being used in an attack to try and lower casualty rates among vehicle units. By the end of the campaign, after their retreat from Dal'yth, the regiment counted in at only 112 Chimeras left of the original 465 that had been deployed, and only 8 of the 45 Leman Russ battle tanks deployed remained.