Vraks
Vraks is a system in the Imperium of Man, near the Eye of Terror. It is (or was) part of the defenses centered on Cadia; in particular, its main planet, Vraks Prime, is an armory world, an entire planet used by the Departmento Munitorum to stockpile weapons and ammunition. The justification is that, if the tithe fleets bringing weapons directly to Cadia from further out in the Imperium get delayed, they can withdraw supplies from Vraks until contact is restored.
As far as the environment goes, it used to be an intensely volcanic world, so there's sulfur everywhere, the weather is stormy and cloudy, and the entire ground is either desert or ocean. The only things living there were algae, and then the Administratum moved in and brought a bunch of humans with them to set up the armory. It eventually got a basilica, to commemorate some martyr or another, and a big fortress to protect the armory (that many tanks and guns in one place proved a tempting target for pirates), designed to be all but invulnerable to orbital attack, and for ten thousand years, Vraks Prime never fell to pirates, Chaos, Xenos, or worker rebellions.
Unfortunately, this state of affairs was not to last, because the Vraks system was created by Forge World for a trilogy of Imperial Armour books; Vraks Prime was soon to be the site of a mighty siege, starring loyalists and traitors. In an unfortunate twist, it was the loyalist Imperials who were on the outside of the defenses.
Fall of Vraks
It was not pirates or xenos that eventually brought Vraks low, but a rogue member of the Ecclesiarchy, one Cardinal Xaphan. Having recently been promoted into the post of Cardinal-Astral of the entire Scarus Sector (if the name is familiar to you, that's because it's where the Eisenhorn and Ravenor trilogies took place, plus it's next door to the Calixis Sector, where Dark Heresy is set. It was named after a Great Crusade era Word Bearer's Captain going missing there ; Captain Scarus - Lorgar, eventually arch-priest of ALL HERESY, bitched out salty tears about this and renamed the sector), Xaphan decided to take a tour of his domain. He found that he enjoyed rousing armies of the faithful, and decided (with some prodding by his number-one oh-so-trustworthy assistant Deacon Mamon) that he could do more good leading an army than working a desk job. The only problem was that pesky Decree Passive preventing him from raising men under arms, so to keep his plans hidden, he went to Vraks, where there were plenty of soldiers and weapons to make an army, as well as a basilica to give him an excuse to stick around.
The Inquisition found out anyway, and sent a Vindicare assassin after him, but the assassin failed; seeing that covertness was no longer an option, Xaphan moved the workers of Vraks to open rebellion. The Imperial presences on the planet, namely the Administratum, Adeptus Arbites, and the Adepta Sororitas (who had been his bodyguards until this point), were either killed or imprisoned, which should have been a clue that he was making a bad move, but Mamon assured him that he was in the right, and that his enemies were merely deluded fools who would see the light once he actually got started on his crusade.
Siege of Vraks
The Departmento Munitorum wasn't going to sit back and let Vraks turn traitor -- not because of the eight million workers living there, but because it was a giant armory (which could not be allowed to fall into traitor hands, or be kept from the regiments who needed them), and because, if word got out, other worlds could fall into rebellion, and use Vraks' supplies for themselves. Therefore, the world would have to be taken, and quickly. The Citadel of Vraks was all but invulnerable to orbital attack, so it could not be assaulted directly (ruling out the Space Marines and Imperial Navy), and the world's massive stores meant that a blockade-and-raid strategy would take five centuries to complete. The only way to take Vraks back in under a century was to put the Citadel under siege and take it by force, so they took some thirty regiments of the Death Korps of Krieg and put them into an army (30 regiments is not that many men, traditional GW oversight. Unless these Regiments had HUNDREDS OF THOUSANDS OF MEN EACH! The Imperium doesn't have a standard size for regiments). They were given twelve years to take Vraks back.
Phase One: Landing and Siege
Year 813.M41. The Citadel was the only thing really worth defending on Vraks, so the planet had no anti-air or anti-orbital defenses on the opposite side of the fortress (which is stupid, there's no excuse to not have anti-orbital defenses across the world, but this is the Imperium of Man so...). The army used the far side of the planet as a landing zone, and over several months, they landed all of the troops, vehicles, and supplies they would need (one could reasonably wonder why other races wouldn't do the same... though to cut them some slack: Vraks' defenses were planned with the idea that they only had to last long enough to get help from the Imperium). Meanwhile, they also built a bunch of rail lines around the planet to carry said army to within a hundred miles of the Citadel's outer defensive lines. Once there, the Kriegers built their own trench and gun network to encircle the fortress. They anticipated that, because the outer defense line was so long, a single massed assault on a particular zone would be flanked and repulsed by the neighbors of their target (a Hedgehog defense), so the plan was to pound the whole thing at once until it folded. They would then surround the second defense line; said line would be small enough to be less resistant to a massed assault, so they would attack a single point to crack the line wide open. Then, they would surround the Citadel itself, and finally be close enough to bombard it directly with their heavy guns and pound it to the ground.
After all of this planning and logistical work, the Death Korps finally began the attack on the outer defense line about a year after first arriving at Vraks, but of course things didn't go as planned. The enemy had substantially reinforced the defenses since the last Imperial survey, and so the "weak point" that the Kriegers tried to attack as a first blow wasn't so weak after all. Matters stalemated for two years, until Captain Tyborc managed to take a bunker whose artillery piece had been destroyed. The guy was a badass even among Kriegers: he took wounds in every part of his body except his left arm, and only eight men of his entire company survived to see reinforcements arrive, but the Imperium had found its breakthrough.
The outer defenses fell pretty quickly, but as the Imperial army got stalled again when the defenders of Vraks made a counter-attack to the North that forced the Imperium to halt its advances in the west and south to get reserves to stop the counter-attack. In the end, the stalemate resumed, just a little tighter around the fortress.
Phase Two: The Noose Tightens
Seven years after the invasion, the Imperial army was still stalled at the second defense line, when reports of renegade Space Marines assisting the defenders started filtering in. It eventually emerged that Mamon was an Alpha Legion spy and had called for backup. Cardinal Xaphan was probably no longer even pretending to serve the Emperor when he signed them up for his crusade. Anyway, two years later, the Imperial army got some more line korps, and to everyone's surprise Azrael showed up with half the Dark Angels in tow as well. His goals were to smash the heretics' star port (pun intended), to stop them from getting more reinforcements, and to capture Arkos the Faithless, the Alpha Legion's commander on Vraks. They successfully captured and destroyed the star port, but Arkos got away and Azrael was severely wounded, not to mention pissed. 200 Dark Angels died in that battle.
The success and the new reinforcements led the Imperial commanders to attempt another massed assault. It stalled (again), until Colonel Attas hatched a plan to have his artillery and infantry make a synchronized assault -- blast the enemy while his troops crawled up through no-mans-land, and then move the bombardment when the infantry was within charging distance, in other words original WW1 Stormtrooper tactics combined with a creeping barrage. Not quite as badass a feat as Tyborc's, but it worked. The second defense line cracked and fell, and the Imperial army was within sight of the Citadel.
Xaphan, having jumped headfirst into heresy by this point, rubbed his hands and cackled with glee, because he knew that the slaughter was only beginning -- he had some more friends on the way.
Phase Three: Chaos Joins the Fray
Captain Fodor managed to put a dent in the inner defense line when he led his company to take a vital bunker. He continued Tyborc's tradition of making footholds with risky, badass feats; among other things, he had a member of his squad torch a trench with a flamer, and then he led the charge into the trench while it was still on fire. Unfortunately, his success was not to last, because Arkos had called in a full Chaos fleet. (At this point, Xaphan had been imprisoned in his own fortress, as he had outlived his usefulness.) They destroyed the Imperial Navy detachment over Vraks and dropped Khorne Berzerkers right onto the inner defense line (Fodor's skull was taken, by the way). Then another ship rolled in and dropped a traitor Titan legion and a bunch of fliers on them. And then some Plague Marines showed up, seized some nasty poison gas stored on Vraks, and gassed a whole regiment.
It was decided that siege tactics were no longer viable. The commander was dismissed (though not executed - nobody blamed him or BLAMMED him for not knowing in advance that so many reinforcements would arrive), and his replacement got some Titans of their own as well as a larger Imperial Navy division to turn the tide back and regain air superiority. They were successful in this, although the enemies didn't break and run this time; instead, they dug in. For those counting, this is twelve years after the siege began.
Phase Four: Breaching the Citadel
Fortunately for the Imperium, the Death Korps of Krieg could fight underground as well as over it. Their miners and engineers worked tirelessly, fought through enemy counter-mining efforts, and finally found and destroyed one of the foundation pylons for the Citadel's inner wall. They destroyed it and brought down a huge chunk of the wall, exposing the squishy insides for the rest of the army to pile through.
The infantry regiments advanced on the breach, first in Gorgons, and then on foot when they reached the remnants of the wall itself (the crater left by the blast was impassable for vehicles). The first attack failed when enemy troops from surrounding wall sections piled into the breach to stem the tide, so a few weeks later, they tried again, with a bigger assault plus a few suppressing attacks on the neighboring wall sections. That failed, too. They tried making more breaches, but while they got pretty good at knocking holes in the walls, they were never able to take them. Fourteen years after the start of the siege (two years longer than planned), the Departmento Munitorum decided that it had worn on long enough, and started to draw Krieg regiments originally destined for Vraks to more successful and more important warzones elsewhere, telling the command staff that they had five years to wrap everything up.
Fortunately (inevitably, for those familiar with Warhammer 40,000 fiction), the Space Marines were on hand to save the day. In particular, the Red Scorpions donated a hundred Marines (from various companies) to the cause. With their Terminators seizing the breach, the remaining Titans gathered up to keep the enemy Titans at bay, and a mixed Marine and Death Korps force following up to hold the ground and drive the enemy away, the Citadel's curtain wall finally fell, and there was only the fortress itself left to take.
Phase Five: Wrapping Up
By this time -- eighteen years after the start of the siege -- word of the extensive involvement of Chaos had reached the ears of the Inquisition, and so Inquisitor Lord Hector Rex brought some Grey Knights and Red Hunters to take control of the situation and get things wrapped up before the Death Korps found itself in over its head. Though the Death Korps was still being drawn down, Rex managed to keep enough soldiers to contain the heretics while he made arrangements to bring a final end to the conflict. First, the remaining artillery brought their combined firepower to bear on the fortress's void shield to overload them. Then, infantry would surge up to take the gates. Tyborc, now a badass Colonel, would lead the charge. The attack stalled, so the Red Hunters made their drop, but the traitors responded with a Reaver Battle Titan and some Alpha Legionnaires, which wiped out the first wave. Finally, a wave of Marauder Bombers cracked the gates and permitted the assault to proceed.
However, Arkos and company had set their Chaotic rituals in motion, and managed to summon An'ggrath, one of Khorne's mightiest Bloodthirsters. He wrecked the entire company of Grey Knights who went into the fortress to stop him, and Inquisitor Rex only barely managed to defeat him. Once he was gone, though, the fight was basically over, and the only remaining task was to clear the catacombs under the fortress.
Some Red Scorpions and the Angels of Absolution turned up to participate in the final attack, to regain the honor of their brothers who had gone before them and snatch up some Alpha Legionnaires for the interrogators, respectively. Arkos killed the Company Master of the Angels of Absolution detachment, but was finally captured (the Interrogator-Chaplain who apprehended him ordered him "disarmed").
Aftermath
The war to retake Vraks took eighteen years instead of twelve, and didn't so much "retake" Vraks as deny it the enemy, since the ammunitions piles and supplies stockpiles on Vraks were completely consumed and those were the primary reason for the siege in the first place. In the end, the primary result was lots of death. The Imperium destroyed fourteen traitor Titans (for the cost of nine Titans of their own), the Dark Angels got a bunch of prisoners from the Alpha Legion to interrogate (in retrospect, their interest may have been due to a possible link between Arkos and the Fallen Angels), and the weapons of Vraks were kept on-planet and not allowed to be taken off-planet by other traitor warbands. On the other hand, Vraks and its defenses were thoroughly ruined by the war, and the contents of its vaults were either used up or so tainted by Chaos that they had to be destroyed. A Pyrrhic victory if there ever was one, the only thing the Imperium gained was being that this world could not be used by the enemy. The prisoners rescued from the cells in the fortress, six Sisters of Battle and Cardinal Xaphan himself (who had long since been turned into a you-know-what), were so destroyed by their experiences that they were barely people anymore. The Sisters ended up in Inquisitorial custody, and thanks to their "care" in Chaos custody, were so experimented on and broken by the time they were freed that they were eventually given the Emperor's Peace (read: a swift merciful death, usually delivered by bolt round or lasblast to the skull), while the thing that used to be Cardinal Xaphan was summarily executed by the Grey Knights who found him.
Meanwhile, Deacon Mamon, the jerk who started the whole thing, escaped justice entirely. In fact, Nurgle was so pleased with how he turned Vraks into a scene of death and decay that he made Mamon a Daemon Prince. Such is life in the grim, dark future.
Tl;dr: Imperium throws millions of guardsmen into a decades long meat grinder and got little to show for it, in fact it would have been more cost effective to exterminatus the damned planet from the begining seeing how the place ended up turning into a dead world anyway.