Originally Posted by Haruspex_Pariah
Did Guilliman force his brothers to adopt the Codex? And did he write it alone?
Sort of, and yes.
The main anti-Codex Primarchs were Dorn and Russ - albeit for two different reasons - and Vulkan.
Dorn was against it because separating Marines from each other was anti-brotherhood, and diluted one of the main strengths of Marines in general.
Guilliman subsequently ordered the Imperial Fleet to bombard Dorn's fleet with torpedoes until he gave in.
/slowclap for Guilliman. In order to prevent civil war, Guilliman was willing to start one. Using non-Astartes military. In a massive
clusterf* of "What is this I don't even..." because Dorn was the one who was supposed to be giving orders with the Emperor (and Horus) down, and Malcador being a pile of ash, so why was the Navy listening to Gulliman?
Russ, on the other hand, took a look around, noticed that the only way they were able to stop
Horus in the first place was because of the massive strengths that the Legions could bring to bear against each other and told Guilliman to pull his head out of his arse, and then congratulated Guilliman for his integral part of the defense of Terra.
Vulkan was also against the Codex, but I can't find a source for why.
Jangatai Khan and Corax (and Guilliman) supported the Codex, again, I can't seem to find why. Khan I can see, but given Corax's actions during and after the Heresy, I can't really figure out why he would support the Codex.
The Dark Angels, Blood Angels and Iron Hands, having no Primarch to stand for them, accepted the Codex with little resistance.
Basically, 6 Legions accepted the Codex. Dorn was bullied into accepting it, and Vulkan (one of the Dropsite Legions) was probably bullied as well, given that there's no chance he could've stood up to a Legion that didn't even fight in the Heresy.
Russ...Was left alone. No-one messes with Russ.
It's just up to each Chapter how seriously they want to take it.
Sort of. Ultimately it's whether or not Terra approves of your Chapter, and, if they don't, what can they do about it, and whether or not it's worth the cost. In broad general terms, the only thing that can kill a Marine is another Marine. And most Chapters aren't willing to fight other Chapters for a variety of reasons.
So Terra using Loyalist Marines to sanction 'Renegades' is usually out of the question unless it's for a really, really good reason (see Badab War), not just because 'they wont follow the Codex'. Which means they use other forces (Sisters, Guard, etc.) which aren't nearly as effective as actual Marines and generally fail at trying to wipe out 'Renegades'.
What was the point then, other than to annoy Dorn?
The main point of the Codex was to thin down individual political power, by thinning down the military might behind it. To Guilliman's credit (and wholly out of character), Guilliman actually did it to himself as well.