Macharian Crusade
The Macharian Crusade was one of the few examples in Imperial History where shit was done and done beautifully by a man with a spirit forged from unbreakable steel: the Lord Solar Macharius.
What makes Macharius so badass? It is because he was just a man. While the Great Crusade had the Emperor, his living demi-god sons the Primarchs and Space Marines beyond Space Marines to conquer the galaxy, Macharius had armies mostly of Imperial Guard and a few Space Marine attachments. And in the space of seven years, he and his armies conquered nearly a thousand worlds. A thousand worlds! That is around one hundred and forty-two worlds being conquered by year. This of course means he and his army were steamrolling all opposition and in a galaxy of xenos and Chaos worshipers, that says much of the man.
It was only when they reached the edge of known space leading into the Halo Zone and the edge of the Astronomican that the resolve of his men wavered. Pushed by seven years of non stop war, they had had enough and refused to go further. Macharius cried manly tears as he rallied against this, for he was having a great time but his men couldn't be persuaded further. And so ended one of the few times in the fluff when the Imperium actually behaved like the greatest power in the galaxy and took territory, rather then just flailing around getting chewed to pieces by heretics and aliens.
Of course this raises a question: if Macharius had won so many planets for the Imperium and was all ready to press on, why did the High Lords not raise another army for him or petition the Space Marines to put together a crusade group for him? It is baffling that they didn't press on their advantage.
Anyway, after the crusade was ended, the Lord Solar Macharius died, mysteriously perishing while back on route to Terra to tell them the full results of his efforts. His end remains shrouded in mystery....which is Games Workshop's way of telling us some Inquisitor decided to off him, as most likely Macharius was actually going back to demand another army to be raised for him and they didn't want that happening.
You see, this kind of infighting and murdering of the best warleaders they have over stupid petty politics is exactly the reason why the Imperium usually doesn't get anything done.
After his death his underlings fought over the worlds he had conquered and made their own private kingdoms, the efforts of a great man undone by a bunch of chattering monkeys, an event known as the Macharian Heresy (even though Macharius himself was about as orthodox as they came). This resulted in the High Lords of Terra having to send in lots of Space Marine Chapters to get things back under control.