Siege of Terra
The Siege of Terra was the end of the Horus Heresy in Warhammer 40,000. If the Horus Heresy can be considered the most important series of events in the 40k universe, then the Siege of Terra itself could be considered the single most important event. It is also possibly the most fucking awesome event; Brother fighting brother, the mighty guns of Titans blowing fortifications to shreds, Imperial Army soldiers leading charges against the traitorous forces even though they know it's suicide and Ollanius Pius
It was Horus's big attempt to off his daddy and so be the true Emperor of the galaxy (for Chaos of course!). He bought a load of his traitor legions, millions of corrupt Imperial army personnel and mutants, the part of the Mechanicus that had gone over to his side and a whole load of daemons to boot. On his side the Emperor had three legions, his custodians and the loyal imperial army regiments of Terra and you know what? The Emperor went and won anyway (granted it was because the Emperor offed Horus before his legions could crack the Imperial palace but still, victory for the home team!).
Horus's side: Sons of Horus; Emperor's Children; Thousand Sons; Death Guard; World Eaters; Night Lords; Iron Warriors; Word Bearers (Though only 5000); Lost and the Damned; several traitor Titan Legions (Mortis, Storm Lords, Flaming Skulls).
(Along with Primarch Vulkan as well as the Knights-Errants)
Duel of the Emperor and Horus
Like with any truly epic event, the siege only ended with the most motherfuckingest duel in the entire 40k fluff; the Emperor of Mankind against Horus, most favoured of the Primarchs and the living avatar of the Chaos Gods. If the Horus Heresy was the most important of a series of events, if the siege was the single most event, then the duel is the defining most of the fluff and affected everything else that came after it.
Although the Emperor managed to win and kill Horus, he was so badly wounded in the end he needed to be on 24/7 life support just to survive. So really when you come down to it it was a draw; Chaos had been stopped then but only at an unthinkable cost to the Imperium.
What, besides the fact that it's the most important event in the 40k universe? Fine.
The Siege of Terra is also the theme for the Horus Heresy board game, in which you reenact the Siege itself. There. Happy?
The fa/tg/uy's explanation of the Siege Of Terra (for Dummies and BL Editors)
The main rule of warfare. As the number of combatants increases, the resemblance to complete uncontrolled insanity approaches infinity. And then you have to take into account the terrain...
Basically you start with a planet that's been nuked, polluted, and generally lived-in for a few hundred thousand years too long (nevermind that this is ~*40,000* years into the future...). And everyone on it is fighting everyone else, constantly. Your basic unit of land is the Bunker, vault 101 style. There isn't any natural plant life left of course, so all the oxygen is made in vats with the food. ""Luckily"" this means you can build anywhere that isn't intensely radioactive, and hence fight over those areas. Get Mega-City-One, nuke it and rebuild it a few times, and then you start to understand.
Then the Emperor comes along, and manages against all odds to conquer the place. Suddenly everyone isn't killing and dying all the time, and a population boom happens. So Emps organised the largest set of public-works since the first colony ships. He rebuilds huge areas of the planet, and creates the Palace, the Astronomicon, and a buttload more besides. And what he gets is effectively one giant city, the second largest (after Comorragh) in the universe. "Huge" just doesn't do it justice as a description. Neither does "Labyrinthine", "Over Populated", or "Gothic Nightmare". And this New Terra was mostly just thrown over the original foundations of whatever was there, like a pile of gold-bricks onto a rat maze. There are bunkers and emplacements still around that date back to the War Against The Men of Iron and even before.
Then the Heresy came, and the Emperor says to Dorn "Fortify this fucking madhouse". So now everything that didn't have a gun emplacement before does now, everywhere. And he walled in half the doors and windows, put hundreds of AA batteries on every roof, and generally panicked because if they would only ever be necessary if the space defenses (the best in the galaxy bar none) have failed.
Then Horus arrives in orbit. He's punched through the space-defenses at massive cost, but the war in space is far from won, and the Palace is just a flat no-fly zone, so he can't just pick and choose landing areas. So he bombards everything his ships can reach, fills the sky with Drop Pods, and tries to march on it.
Which is when Rule #1 kicks in and everything immediately gets Mega-Fucked, for both sides. Ruined streets and bunkers make navigating a nightmare, communications are somewhere between impossible and actively detrimental, Drop Pods land off target, plans and back-up plans fall apart in seconds, demons run amok. And the Primarchs are either constantly trying to out Tactical Genius each other or are too in the thick of it to relay any commands, so no-one has a fucking clue what's actually going on in the big picture. It's Stalingrad writ large, but without even the merest hint of sanity, and a thousand space marines charging into every breach. The inclusion of cackling daemons, rampaging renegade guardsman, and bellowing daemon engines doesn't help the situation.
So it takes roughly 10 minutes of this menial bullshit for a load of the Chaos forces to get bored and just decide "Fuck It, Let's Just Wreck The Place". So now everything makes even less sense, entire companies ignoring sensible objectives to go on the Chaos Marine equivalent of a bender. The Emperor's Children and Night Lords rape, murder, and pillage the civilians of Terra so hard that even 10,000 years later they still live in fear at the memory, while the World Eaters hacked and slashed at anything they thought might bleed. Only the Iron Warriors and Sons of Horus are whole-heartedly tearing at the palace, dedicated to rubbing it in Dorn's face like a bitch no matter what. And to the horror of the loyalists, they're succeeding. Brick by brick the greatest military stronghold in the galaxy is falling.
Which sounds great for Chaos, were it not for the simple fact it wasn't falling quickly enough. It was taking days to advance inches at massive cost, and Guilliman was en-route with reinforcements, with Russ, El'Jonson alongside whatever remained of their respective Legions right behind. If the siege wasn't ended before they got there, the traitors would likely lose. So Horus put all his cards on the table and lowered his battle barge's shields, goading the Emperor (who didn't know about the reinforcements) on-board to hopefully kill him and force the defenders to rout. Everything else is history.