Codex Astartes

The Codex Astartes was written by Roboute Guilliman shortly after the Horus Heresy; among other things, its suggested organizational scheme split the remaining Space Marine Legions into small, manageable chunks. It also includes tactical doctrines for just about every situation, like the formidable Steel Rain. Most loyalist Space Marines follow the Codex Astartes to varying degrees. The Ultramarines and their successors follow it like the holy writ, while the Space Wolves only find it useful for toilet paper.

Marine roles

Among the tenets of the Codex, Guilliman outlined the roles of various Marines to be used in Chapters. They include the follow:

• Assault: Assault Marines are those who specialize in melee combat, using Chainswords, Thunder hammers, and Power swords alike to fuck up the enemies of the Imperium of Man. In addition, they also possess jump packs which can allow them limited flight.

• Devastator: Devastator Marines can be summed up in three words: Blow shit up. The purpose of the Devastator is to provide heavy weapons fire in battle, artillery strikes, and hunting tanks.

• Scout: Scout Marines are newly initiated Space Marines who are charged with scouting terrain and enemy positions. A risky job, but it they're killed, the enemy probably did the Chapter a favor by killing them so quickly.

• Tactical: Tactical Marines are those who are flexible in combat, relying on both ranged weapons and melee weapons in combat.

• Veteran: Veteran Marines are those who have seen through several centuries, allowing them all the cool toys in the Chapter's armory. Most chapter veterans get to wear Terminator armor, but they also sometimes get Artificer power-armor instead.

Chapters

Under the Codex Astartes, the Space Marine legions were split into one thousand strong Chapters composed of ten companies. The first company is composed of Veterans, and are the only company in a chapter to employ Terminator armor.

The second, third, fourth, and fifth companies are called battle companies, composed of six Tactical Squads, two Devastator Squads, and two Assault Squads.

The sixth, seventh, eighth, and ninth companies are the reserve squads, and are more specialized than the first five companies. The sixth and seventh squad are composed entirely of Tactical Marines, the eighth company of Devastator Marines, and the ninth company entirely of Assault Marines.

The tenth company is composed entirely of Scout Marines, who are eventually promoted to other companies after a certain number of years.

Dubious helpfulness of the Codex

While the Codex Astartes has a few good points on how Space Marines should fight, it contains a truckload of a lot of bad points on even how an army should be. Due to Guilliman's unending paranoia over the fact that none of his brothers are possibly loyal anymore (and to cover up on how fucking useless he was during most of the Horus Heresy), he made it so that he ruined the Space Marines entirely, which in comparison is oddly prophetic to this man who worked on the Space Marine codex and butchered it with Ultramarine fanboyism.

One fatal flaw of the Codex is the splitting up of the legions and maintaining a maximum of 1000 battle brothers at any given time, this made the Space Marines even more of a fragmented force concerned with their own agendas than a unified fighting force along with making it longer for them to rebuild and reform whenever they sustain heavy casualties after a major campaign.

The other flaw was turning each chapter into a generic army using a different name. The Codex Astartes details specifically how a Space Marine should act in a combat situation, ranging from engaging Orks to the Tau. While this does give marines a good idea on how to act when they're staring down a battlesuit gunline or a marauding pack of Kroot, this makes them far too predictable for those who have faced the Space Marines for a long time. As Sun Tzu said: "Do not face your opponent too many times or you will reveal your art of war", this basically means that an intelligent opponent with enough dakka to beat back marines will quickly realize how to deal with any marine force strictly adhering to the Codex Astartes' tactics This along with the fact that the codex does not detail how to deal with every murderous creature in the galaxy which the Imperium has yet to discover, preventing marines from quickly formulating and adapting for different tactics not mentioned in the Codex, which is greatly exemplified when the Ultramarines got their asses handed beaten hard when they faced the Tyranid incursion in their sector for the first time. This all in all impairs the capability for Space Marines to be flexible at any given time since they have to adhere to a stringent rulebook, or else.

In comparison to chapters who go off doing their own thing like the Space Wolves and Black Templars, and those who do not fully embrace all of the tactical doctrines of the codex like the Dark Angels and Blood Angels, they have ironically fought and won more battles that shaped the history of the Imperium and WH40K than the Ultramarines and their successor chapters, who strictly adhere to the Codex: Astartes down to the letter. This basically exemplifies how utterly bollocks the Codex Astartes is, and while it should be a suggestion on how marines should act, it should never be the rulebook.