Astra Militarum - Era Indomitus: In the period now called the Era Indomitus, Humanity's response to the cataclysm wrought by the birth of the Cicatrix Maledictum in the wake of the 13th Black Crusade and the Fall of Cadia -- the outpourings of Heretic and Traitors, the surges of mutation and xenos aggression -- was not the spiteful lash of a dying beast. It was the roar of a titan roused to the warpath and its manifestation was arguably greater than the mythic battle groups and armies said to have been loosed by the Emperor Himself during the Great Crusade.Millions of guardsmen had fallen on Cadia alone. Innumerably more were wiped out with the tearing open of the Great Rift into the Immaterium and the concomitant eruptions of Warp storms old and new, or perished fighting the hellish enemies that had boiled out of their depths. The fate of half the Human-settled galaxy -- designated the Imperium Nihilus -- remained unknown beyond the impenetrable wall of the Great Rift, while threats of every conceivable evil assailed thousands of star systems within the other half of Imperial space where the light of the Emperor's Astronomican still shone -- the Imperium Sanctus.When the Great Rift tore open, it unleashed violent tidal waves of empyric energy that battered numerous Imperial war fleets into oblivion. Entire war-torn star systems were swallowed by the Warp. Newly-raised regiments on highly-tithed worlds; industrial planets preparing requisition supplies; troop transports bearing elite battalions to embatted war zones -- at a stroke, whole consignments of materiel and desperate-needed soldiers were wiped out.Worse was to come. In the darkness and isolation that followed as the Astronomican was blotted out -- the so-called Noctis Aeterna -- foundering fleets lost their way as their Navigators lost their ability to find paths through the Immaterium and astropathic pleas for aid went unheard. Numerous Imperial Guard army groups thought to have survived the initial apocalypse were later deemed dead, lost or worse. Imperial forces upon surviving worlds or in the holds of intact transport ships were preyed upon by Heretic fleets and Daemonic legions that surged out of the churning Warp storms. Xenos raiders, some refugees fleeing the effects of the Great Rift's birth themselves, opportunistically tore into weakened Imperial garrisons and pounced on cut-off companies and regiments.When fitful glimpses of the Emperor's light in the Astronomican did eventually return as the Empyrean calmed, they illuminated a ravaged Imperium. Screaming telepathic messages flooded the suviving astropathic choirs and data churned through overwrought cogitators as the Departmento Munitorum struggled to collate the extent of the damage.When the Primarch Roboute Guilliman mustered his Indomitus Crusade, yet more armies were stripped from already beleaguered Lord Commanders even while they vowed to stand firm. Yet for the myriad armies of the Astra Militarum, the dark times were only just beginning.Everywhere, it seemed to the overburdened staff officers and lords general of the Astra Militarum, war zone commanders demanded urgent logistical support and mountains of additional ammunition, cried out for desperately needed reserves or, worse, fell omninously silent. Tithes were increased on already suffering worlds. Vicious conscription drives emptied entire hive cities. Heightened output needs pushed Forge Worlds and Industrial Worlds to their limits in desperate attempts to make up production shortfalls. Each Militarum Regimentum -- the total armed forces of a given world that fall under the remit of the Astra Militarum's tithe -- was stretched to breaking point, their individual regiments needed as never before.Roboute Guilliman's Indomitus Crusade was but one answer to the hurricane of woes that daily beset him as lord commander of the Imperium and Imperial Regent. Between Guilliman, his senior commanders and his army of bureaucrats in the Officio Logisticarum -- the competitor to the Departmento Munitorum Guilliman created to circumvent the Imperium's increasingly sclerotic bureaucracy -- the herculean effort of gathering the crusade fleets began. To secret mustering points throughout the Sol System and beyond, thousands of regiments converged, joining with Space Marines, Sisters of Battle, the cybernetic hosts of the Adeptus Mechanicus and more -- yet this was only the beginning.Each fleet's battle groups, once launched, fitfully made their way through the churning Warp on predetermined courses. Where they came to state systems miraculously spared carnage, they each reaped a harvest of yet more regiments. Where their task forces found worlds in the grip of Chaos or xenos invasion, the Imperial Guard's regiments of massed infantry, battle tanks and artillery almost always formed the vast majority of every crusade force. In the wake of each victory, reinforcements from the recently saved world were demanded to replenish the ravaged ranks and replace wrecked armoured vehicles.The fleets of the Indomitus Crusade continue Guilliman's vision of pushing back the corrupting armies of Chaos and those enacting xenos tyranny. Guilliman's objectives stretched beyond mere attack, however; his Officio Logisticarum was granted the authority to requisition tithed regiments for the protection of its own adepts and their hub-fortresses, and senior Astra Militarum officers were ever amongst the luminary nobles and warrior-generals assembled by the Indomitus Crusade. Several were appointed by Guilliman to command entire battle groups and even fleets, such as the dynamic leader of Fleet Tertius, Fleetmaster Cassandra VanLeskus, formerly a hereditary general -- or markha-gher -- of the Vodine Sergastae regiments.The demands the Indomitus Crusade placed upon the Astra Militarum were unparalleled. As the battle groups moved from system to system, they subsumed innumerable newly rrused regiments to replace their losses and swell their combat assets. Many regiments were also deployed to the Officio Logisticarum's hub-fortresses in the crusade's mustering sites, and commanders of the crusade were instructed to found and man new hub-fortresses in conquered systems.Guilliman's ambitious aims for the Indomitus Crusade were vital for the Imperium's future, but the battlefields its fleets were destined for made up only a fraction of the war zones the Imperial Guard fought in. Within thousands of sub-sectors that lay nowhere near the path of any Indomitus Crusade battle group, Astra Militarum forces were dispatched to new or escalating war fronts on a daily basis, coordinated by the masters of localised Departmento Munitorum officios and their armies of scribes, adepts, logisticians and strategos. Infantry regiments, armoured battalions, artillery divisions and super-heavy tank companies were constantly deployed and redeployed, reinforced, consolidated or evacuated. The grinding logistics involved in moving huge numbers of troops and materiel through storm-tossed Warp channels only compounded this undertaking, adding its own dangers anddelays.Xenos invasions were confronted: Orks and Tyranids in the Octarius Sector; Necrons in the Erinaeus Cluster; Aeldari raiders in the Desh'ka Sub-sector, to name but some. Multiplying Chaos Cult and Genestealer Cult uprisings were crushed and uprooted, some in thrall to the Dark Gods of Chaos, others bearing xenos-implanted mutations or ideas, or fighting for secessionist tenets fuelled by political or religious heresy.Only three of the five Lord Commanders of the Segmentae Majoris were in tentative contact with the wider Imperium during the Era Indomitus. Of these, Arcadian Leontus, the Lord Commander Solar, was arguably the most embattled. The Segmentum Solar -- home to Terra itself -- was assailed from without by Heretic fleets and countless xenos, and from within by insurgents and Traitors. Leontus refused to allow such threats to come to fruition under his watch, but did so by expanding his remit beyond the Segmentum Solar to anywhere he deemed their to be a threat that might reach the Sol System itself. His efforts were to prove crucial in the galactic east during the horrors of the Fourth Tyrannic War unleashed by the Hive Mind of Hive Fleet Leviathan in an attempt to take advantage of the Imperium's preoccupation with the struggle against Chaos by assaulting the Emperor's realm from the opposite direction of the Indomitus Crusade's primary efforts.