The Space Marines were the lauded heroes of the Great Crusade, before ending up mortal enemies during the battles of the Horus Heresy.
We know the Space Marine Legions were big – they had to conquer Terra and then the entire galaxy, after all. Many sources have alluded to their scale and disposition, and fans are often understandably curious about exact numbers. But the truth is often hard to pin down.
The Space Marine Legions that set off from Terra in the early days of the Great Crusade numbered at least 100,000 each, far larger than the Chapters of the 41st Millennium.
But that alone was still not enough for the task of subduing the Galaxy. In this optimistic era of reconquest, Space Marines were rarely deployed to a combat theatre alone. They were the tip of humanity’s spear, the shock troops who shattered the resolve of enemy empires, crushed Xenos tyrants and blazed a trail for the more numerous but slower forces of the millions-strong Imperial Army.
As the Legions were slowly reunited with their Primarchs, their composition changed, including their overall numbers. Some grew swiftly, white others favoured a more compact organisation – and they all came to reflect the cultures of their adoptive homeworlds and preferences of their gene-fathers.
Some of these changes in size were well recorded, while others were only ever estimated. With many Legions fighting across multiple warfronts, and recruitment largely their own specific purview, no single source can be 100% accurate.
Some Legions stuck rigidly to their original structure, like Imperial Fists, ever keen to uphold the Emperor's original vision.
Others were known to have grown significantly. The Ultramarines were much larger than the other Legions by the time the Heresy broke out, while the Sons of Horus and Word Bearers were also numerous. The Iron Warriors rivalled the Ultramarines in size, with pre-Heresy estimates putting that Legion at 180,000 warriors.
Many Legions were smaller as much by attrition as any logistical choice. The Thousand Sons and Space Wolves had tumultuous histories even before the Horus Heresy, and both were reduced further by their tragic conflict, engineered by the Warmaster.
The Space Wolves deployed 75,000 troops to this battle, yet only a third of that number survived by the time the planet burned – where that left their overall strength, however, is anyone’s guess. The Thousand Sons fared even worse, though no imperial record can say how many escaped the wrath of Leman Russ.
The Dark Angels entered the Heresy in reduced numbers after a bruising campaign against the Xenos Rangdan. These bloody battles of extermination laid low almost an entire Legion’s worth of Space Marines, a good number of them sons of the Lion.
The size of some legions can only be guessed at. No definitive records exist of the White Scars, Night Lords, or Alpha Legion*, though we know that at least 50,000 of Alpharius’ sons were present at the Dropsite Massacre.
Traitor numbers at the start of the Horus Heresy are harder to be sure of, as most had undergone a period of “purging” Imperial loyalists from their ranks. The Death Guard numbered around 95,000 by the end of the Great Crusade, for example, and are estimated to have culled about a quarter of their strength on Isstvan III. The World Eaters, Sons of Horus and Emperor’s Children were similarly purified of dissent.
Many Legions shrank quickly in size as the brutal attrition of the Horus Heresy inter-Legion war began. None more so than the three Loyalists at the Dropsite Massacre. The Raven Guard and Salamanders deployed in almost full strength, and most were slain. The Iron Hands, meanwhile, were present in lesser numbers, but deployed their most elite companies, almost all of whom perished alongside their Primarch.
Despite this, all three of the so-called Shattered Legions retained enough their strength to become a thorn in the side of the Traitors even as they marched on Terra.
As the Horus Heresy ground on, many legions accelerated their recruitment rates, struggling to match the constant rate of attrition, so brutal were the battles of the Age of Darkness. By the Siege of Terra, most Legions had resorted to rapid indoctrination processes, swelling their ranks with thousands of untested but battle-hungry Inductii. These warriors were still Space Marines, but to them the glories of the Great Crusade were already half-understood legend. They lived only to fight and die in a hate-filled civil war against other Space Marines.
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