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 holy writ, while the Space Wolves only find it useful for toilet paper.
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. They train together, and when a Space Marine Company deploys as a single unit, it will be one of these.
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 (who specialize in using bikes and Land Speeders, respectively), 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.
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.
Ranks
The Codex Astartes formalized and simplified the ranks among the Adeptus Astartes, eliminating some ranks like commander and lieutenant, while adding some more. They include:
- Chapter Master: The grand poobah of the Chapter. He is the one who gives Marine companies their assignments, and ultimately recognizes no authority save for the God-Emperor of Mankind.
- Captain: Captains are the commanders of Companies, overseeing their Marines from the front line instead of the back. Captains are usually promoted from the senior Sergeant in the Company. Each Company Captain also holds a Chapter-level position of authority, supervising his Chapter's fleet, vehicle pool, recruits, or territory, for example.
- Sergeant: Sergeants lead squads of four to nine other Marines. There can be anywhere from ten to twenty Sergeants in a Company.
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 every combat situation that Roboute Guilliman had encountered. 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;" an intelligent opponent with enough dakka to survive one encounter with a Codex Chapter will quickly realize how to deal with any Marine force strictly adhering to the Codex Astartes' tactics. This fault is compounded by the fact that the Codex has no information on threats encountered after Guilliman's time. The Ultramarines experienced this personally when they got their asses beaten hard when they faced the Tyranid incursion in their sector for the first time, and when one of their Captains developed a novel set of tactics for dealing with them (successfully), he was punished for deviating from the Codex!
Compare this to chapters who follow their own standard operating procedures, 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 and their successors have a much more extensive list of victories, and a shorter list of failures. As in all matters, moderation is key: Guilliman was no fool, but he was not omniscient, either.