Indomitus Crusade

"To the Space Marine Chapters bloodied and besieged in the Dark Imperium, we bring reinforcement. To the Chapters lost in valiant duty or driven to destruction, we offer rebirth. To the enemies of my father's empire, we bring death. With these words I, Roboute Guilliman, Lord Commander of the Imperium, son of the Emperor of Mankind declare the Indomitus Crusade. Traitors, mutants, daemons. Heed to the coming of my armies and the ruin we promise to your miserable kind. This galaxy is ours!"

– Roboute Guilliman, Lord Commander of the Imperium, son of the Emperor of Mankind.

The Indomitus Crusade is the Imperium's latest major military campaign, in response to Abaddon's 13th Black Crusade, its successful destruction of Cadia and the fracturing of the galaxy in two pieces. After the combined effords of Archmagos Cawl, Saint Celestine and the Ynnari to bring the Primarch Roboute Guilliman back from stasis, he reaches Terra and takes full control of the Imperium to try and get shit done. To do this, he declares a massive crusade to push back the forces of Chaos, unveiling his brand new and very marketable Primaris Marines, to spearhead his attack against Chaos, even if no one really trusts these new guys. The Indomitus Crusade lasted around a century and managed to stabilize the Imperium, but the 13th Black Crusade is technically still going on. Also, this event is the first major step into the 42 Millenium (even though technically could not be the case because calendars are fucked in this grimdark future), Ciaphas Cain's novels notwithstanding.

This event is specially remarkable because this is the biggest advance of the 40K storyline in decades, and making people really afraid of this being another Age of Sigmar. For now, those fears have been pushed aside, the galaxy is still there (although split by the Cicatrix Maledictum). It also brought forth the 8th Ed Codex, which focuses a lot on the battle of the Imperium vs Chaos (so nothing has changed that much, I suppose).

Phase 1: Gathering forces and the Second Battle of Terra

Soon after Guilliman arrived to Terra and took the title of Lord Commander of the Imperium (basically becoming the de facto ruler of the Imperium of Mankind), he started to gather up all the forces he could to counter the forces of Chaos that were pouring from the Warp in massive amounts. Taking all Astartes, Mechanicum, Navy, Guard Regiments, the Sisters of Silence, some Custodes elements (finally entering the galactic fray after millenia of doing fuck all), and the new and now on sale Primaris Marines to spearhead the counterattack. While he was busy gathering his forces, however, Terra was attacked by the Legions of Khorne. The Blood God was fed up with the other idiots mucking about around the Cicatrix Maledictum believing they'd won, so he went to finish the job on his own and sent in eight of his Legions... who were quickly and swiftly sent running back to the Warp with extreme prejudice and overwhelming firepower.

This small incursion, however, caused massive panic and unrest on Terra proper. The Terran anti-Warp defenses failing to prevent the opening of a rift from where Chaos attacks could pour from was a major thorn, and some argued that Guilliman should stay to defend Terra. But the Primarch knew that to prevent the situation from going completely bad, he had to bring the fight to the enemy. His forces set out from the centre of the Imperium, course set for the hundreds of planets either under siege or already conquered by the Ruinous Powers. During all of this, the Astronomicon was not working properly, constantly flickering and making Warp jumps a pretty risky move. And considering Guilliman's Crusade used almost half of the Navy Fleet in Segmentum Solar, they had to do very short jumps, slowing the beginning of the Crusade down considerably.

Phase 2: Guilliman frees a bunch of planets and brings Primarines to the Chapters

Guilliman soon arrived into the thick of things. He travelled around dozens of planets, liberating those that could be saved and exterminating those that could not. Slowly but surely, Imperial forces managed to push back the chaotic tide, although it was only enough to stabilize some of the war fronts. The arrival to Catachan was a specially interesting point in the Crusade: the planet's local fauna was so deadly it scared away even the daemons, which probably brought a decent amount of morale uplifting laughter to the Imperial forces involved. As the tides of the Warp calmed, astrophatic communications started to improve all around (even if the Imperium Nihilus half was still pretty much isolated from the light of the Astronomican). The more planets the Primarch freed, the more reinforcements he added to his campaign and the more momentum it built up.

It is also during this phase that the already extant Astartes Chapters started getting their Primaris reinforcements, with all manner of colourful reactions. After much grumbling, the general consensus ended up being "accept them now, complain when the war is over".

Phase 3: Nurgle attacks Ultramar and Guilliman ends the Crusade

After a century of war and freeing planets beyond count ( including Baal from the Khornate daemons AND Hive Fleet Leviathan, poor Blood Angels can't catch a break. By the time Rowboat Girlyman showed up Dante had just solo'd the swarmlord. All the heavy lifting was already done by the Blood Angels.), the front was largely stabilized, Chaos contained into the Cicatrix Maledictum and tentative contact had been reestablished with Imperium Nihilus. Guilliman's forces were however exhausted and Belisarius Cawl needed to refill the Primarines' ranks. Many of the crusading forces wanted to consolidate their gains rather than continuing the conquests and stretching their forces thinner and thinner throughout the galaxy.

The deciding point was the invasion of the Ultramar realm by the forces of Nurgle, taking advantage of the drawing of forces for the Crusade leaving planets relatively undefended. Thus, Guilliman called it a day and dissolved the Indomitus Crusade, allowing everyone to go home and deal with the invasion. With that, the Indomitus Crusade reached its end on a globally positive note that the 13th Black Crusade had been mostly contained and that the situation was no longer deteriorating quite as badly as it was before.

Results of the conflict

The Indomitus Crusade managed to push the forces of Chaos long enough for the 13th Black Crusade to start losing its momentum (because Chaos doesn't stay organized for long and a lot of its warlords decided to start pillaging nearby systems rather than keep pushing towards Terra), thus preventing more damage from being done. This, however, doesn't mean that the Imperium didn't suffer a massive blow: The Cicatrix Maledictum and the losing of planets in hundred thousands means this was the biggest blow the Imperium has suffered since the days of the Horus Heresy or the War of the Beast.

The Dark Imperium Forms

Half of the Imperium still gets barely any light from the Astronomican, isolating the Imperium Nihilus from the Emperor's light (and also cutting it off from any reinforcements that might come from the other side of the Imperium, except through one available warp route), exposing it to future attacks from heretics and xenos alike, and making warp travel an even more hazardous experience. The splitting of the Imperium into two halves (one that is able to keep itself stable enough due to Guilliman and being on the same side as the Astronomican, and one which is basically daemon food) is bound to cause problems on the off-chance there's enough of the Imperium left to break away in the Imperium Nihilus. Not to mention that most Chaos forces are still running amok throughout the galaxy and now have a gigantic base of operations to launch their attacks from instead of the much smaller Eye of Terror and the Maelstrom.

Other Imperial issues

The Primaris Marines debate has only gotten worse after the end of the Crusade, most people seeing Cawl's creations as either necessary abominations or an outright heresy, depending on whom you ask. Which, if Guilliman speaks soon and intelligently, can be dismissed by him pointing out the Primaris are made from the most pure gene-seed in a device created by the Emperor himself and also uses techniques used in the creation of the Custodes. So, to call it heresy is like saying the Primarchs and Astartes in general are heretical as the Primaris gene-seed is both more pure than that of the normal Astartes stock and directly created by the Emperor personally.

On the adminstrative side, Guilliman has to manage several warzones at once while ruling the whole Imperium and dealing with the various groups that are opposed to his attempted reforms, which is taking a toll on the Primarch. The Mechanicus's higher echelons are also highly annoyed that Cawl is calling so many shots, and some of them have even accused him of techno-heresy due to his highly unorthodox views on innovation. Which is funny because he was a key part in creating the Primarines and their armor so anything he does with it can’t really be considered heresy as the Emperor gave him leave to manipulate Astartes genetics to do his job in the first place before the Great Crusade.

Xenos

Meanwhile, the Tyranids ate some more planets and are pushing further into Segmentum Solar, the Tau lost a whole expansion sphere when they ran straight into the Death Guard's main force, the Necrons expanded some of their empires, the Orks did what Orks do (hint: it involves Waaagh!, choppa and dakka) but they're more organized now, and the Craftworld Eldar are trying to figure out what to think of the Ynnari while generally attempting to not get wiped out by Chaos. The xenos and the danger they pose haven't gone anywhere, but for the time being the Imperium considers Chaos to be the more immediate threat to its existence.