Was a legion's philosophy and tactical preference inherent even before being reunited with their Primarch?

Question

The legions were created by the Emperor out of the genetic material of each Primarch, but in all cases the Primarchs were also shaped and changed by their experiences growing up and the environments they found themselves in. For instance, Corax learned the art of guerrilla warfare and hit-and-run tactics, Lorgar became a religious fanatic, and Leman Russ became more feral.

But is there any indication that the legions already had these elements in them before they met their respective Primarchs? Were the Iron Warriors and Imperial Fists always siege masters, did the Death Guard always have a higher than average resilience, did the Emperor's Children always seek perfection and appreciate the arts? Or did the legions only get their characters after meeting their Primarchs?

Answer 1

This answer will need significant expansion to cover all 18 Legions. But yes, certain aspects of a Legions character were already fixed in alignment with their Primarch's fate, even before the Primarch was found by the Emperor.

V Legion : White Scars

. Their early experience in pioneering and scouting also saw the Star Hunters master the art of hit-and-run and maneuver warfare that would come to define them in the future. Soon enough the Star Hunters were known not only as pioneers but also as raiders who struck swiftly and without warning.

The hit and run tactics favoured by the Star Hunters prior to their reunion with their Primarch Jaghatai Khan mirrors the hit and run tactics used by The Khan in the defeat of the Palatine's Empire by the tribes of Chogoris and the future White Scars they would become.

Initial recruits for the Star Hunters were drawn from the ice wastes of the Thulean Basin. While this isn't a duplicate of the region of Chogoris that Jaghatai was raised on, the barren grasslands of the Empty Quarter and the Icelandic wastes share many similarities in the difficulties they pose towards those living there.

VI Legion : Space Wolves

Before the coming of their Primarch Leman Russ, the Space Wolves were known as the VI Legion and was made up of a diverse range of savage and hyper-violent tribesmen from Terra.

The background of the VI Legion is a strong match for the savage tribes of Fenris where Leman Russ is found, and eventually becomes the chapter's homeworld.

VIII Legion : Night Lords

Originally known as the VIIIth Legion, the Night Lords first recruits were from the stinking ancient prisons of Terra. Here the children of prisoners were raised in the dark and among death. These pale "Night's Children" made perfect Astartes recruits.

This is uncannily similar to Konrad Curze background on the lawless hive planet Nostramo.

XV Legion: The Thousand Sons

Five years into the Crusade however, the warriors of the legion – much to their delight - all began to spontaneously develop psychic abilities.

Over the next several decades some of the then-discovered primarchs found the notion of psychic mutants being allowed to exist as part of the Emperor's Crusade distasteful at best...

It was shortly after this that the Great Crusade arrived at Prospero and found Magnus...

The Thousand Sons Primarch is of course, Magnus the Red, found on Prospero, a world of outcast human psykers, where Magnus' own psychic abilities were allowed to grow and florish.

XXth Legion: Alpha Legion

Like the Salamanders and Space Wolves, the XXth Legion was formed and established largely in separation from the rest of the Legiones Astartes, likely for a very specific purpose. These three specific legions were obliquely called by some sources as "Trefoil".The earliest known activities of the legion saw it conduct abductions, targeted strikes, and assassinations to the Imperium's enemies on Terra and beyond.

While the "secretive" birth that the Alpha Legion shares with its mysterious Primarch, is shared by two other Legions. The role it expanded into enhanced this secrecy more than was true for the others. Alpharious's story does not include a home planet, but his first encounter with the emperium was marked by similar tactics of a small force vs a more numerous foe;

Using a ragtag fleet of primitive one and two-man fighters he cleverly put the advance ship of the Luna Wolves' fleet in a precarious position, which necessitated the intervention of Horus himself.

The XXth's signature methods fit perfectly with their Primarch's approach to warfare.

Alpharius quickly developed a unique approach to Astartes operations, focusing on the philosophies of initiative and flexibility, as well as extensive use of subterfuge and non-Astartes specialist operatives.

Answer 2

The gene-seed used to create the space marines was created using genetic material from their relevant Primarchs, so you could think of the Marines as being children of the Primarchs.

Also, as some of the gene-seed organs affect the brain it is reasonable that the marines will have some behavioural characteristics to match their Primarch.

All of the gene-seed was originally crafted in the 30th Millennium by the Emperor of Mankind in His gene-laboratories deep beneath the Himalazian (Himalayan) Mountains for this purpose using the genomes of each of the 20 Primarchs as a template.

Gene-seed

Answer 3

To a certain degree, yes with an exception

Almost every legion showed signs and lesser forms of what they would become in the later stages of the great crusade and after their reunion with their respective primarchs. The Warhounds were already considered to be unusually aggressive, the Salamanders fatalistic and the sons of Perturabo were already known for siege warfare.

exceptions

The Dusk raiders(Death Guard) drastically changed their legion philosophy once Mortarion joined, clearly felt through the legion with the shunning of the original members as time worn on. Their tactical preference changed from raiding surprise style to a slow and inevitable death for their foes.

It is also worth noting that even though the salamander style did not change overly much they did become less fatalistic once reunited with Vulkan who tempered the legions spirit.

Definitive increase

Not counting the exceptions, almost all legions previously mentioned straits got turned to 100 when reunited with their primarchs, the space wolves developed even more physical traits, the Ultramarines got obsessed with rigid structures(tacticals and practicals), the sons of Dorn become almost entirely focused on siege related matters, the raven guard increased their focus for guerrilla warfare and the s̶p̶e̶e̶d̶ ̶f̶r̶e̶a̶k̶s White Scars really adopted the Chogorian culture

Answer 4

It is a "nature vs nurture" question - i.e. was Corax "sneaky" because he was born this way (hence all Raven Guards are sneaky on genetic level) or was he made this way by his upbringing (so the original RG marines had to be trained to be stealth specialists)? Was Death Guard inherently tough or just made this way after Mortarion was toughened-up by growing up on a poisoned planet?

The answer is most likely "we don't know". However, we can speculate:

If the link between marines and their primarch is most likely stronger than mere genetics (i.e. by some sort of a warp-link), then the brand-new legions already had some of the "gained" properties of their gene-fathers.

But even more likely is that the Emperor had a vague knowledge about his primarchs situation (and almost certainly he knew what sort of warrior would each of primarch become if he wasn't stolen by warp) and he was creating the new legions to match it, for example by selecting the toughest candidates to the Dusk Riders (future Death Guard) legion.