ItsUncertainWho said:

In that case it sounds like Guilliman should've been made the Warmaster since I kind of thought that the Warmaster was supposed to handle the large picture and delegate tasks to the most suitable subordinate commanders. To me it seems that even if the Lion would be the best tactician, maybe he didn't see the big picture clear enough and that's what made the Emperor pass over him?

Gurkhal said:

According to Lexicanum that's precisely what made Horus a great Warmaster, being able to put the right forces in the right place.

The Guilliman answer to that would be make sure they have the right amount forces in the first place, something that cannot be understated. It's that what caused the Department Munitorium to be created.

Possibly the Lion was the best mix of defensive and offensive tactics, possibly he was not as charasmatic a leader as Horus who could inspire miriad of forces to work together (against the Emperor in the end sadly).

Gurkhal said:

Not really.

My view leans more to the idea that Guillimans flaw is he won't take risks unless it's a last resort. He would rather grind out an engagement if he can guarantee a win in the long run instead of a quick, dirty fight that will accomplish an objective but might leave something unfinished. Guilliman is playing the statistics instead of instinct. To an extent I see him as overly cautious when compared to other Primarchs. He won't act until everything is in place and nearly perfect.

Horus delegated well. Based on the HH novels, he might have delegated too much and ended up allowing his advisers to blind him. The Lion was too concerned with what was in front of him, the crusade, to notice what was going on behind him, on Caliban. Alpharius was caught up in being clever and discounted that Guilliman new his enemy and adapted accordingly. All the Primarchs have flaws and strengths.

We need to keep in mind that when we start talking about one Primarch being better than another at something, we are talking small degrees. The only thing the Primarchs truly have in common is that they are all bitchy little emo kids complaining that daddy doesn't pay enough attention to them.

Really, there were only two choices for Warmaster. Horus, who had the skills as a diplomat, who could actually convince the other primarchs to work together for a greater cause. Horus would have LOVED the Tau... you know... before destroying them without remorse.

The other option was Sanguinius who could inspire hope and the sense of greatness in others. You didn't fight because Sanguinius told you to, you fought cause it make you feel like a better person, like you were choosing a higher path.

Being the Warmaster had nothing to do with strategy - it had to do with herding the superpowered cats that were the Primarchs. There's no way Gulliman could have pulled that off. Too blunt, too many enemies. Could you imagine Russ, Perterabo, Alpharius or Fulgrim taking orders from Guilliman? Do I even have to mention Lorgar? None of the Primarchs had a huge beef with Horus. None of them had anything against Sanguinius. THey were really the only choices.

Okay, now I want to steal the Dornian Heresy idea and put it in place to design the Sanguine Heresy - the fall of Warmaster Sanguinius...

Regarding Guilliman and being Warmaster, I don't think it's all that clear-cut.

All the Primarchs are military geniuses in every sense of the word. Differences between them are matters of degrees which are only visible when comparing Primarch to Primarch.

That said, Horus himself mentions in Horus Rising that he believes that Sanguinius should have been Warmaster. Guilliman muses to himself in Know No Fear that only he, Horus, Dorn and Guilliman were viable choices amongst all the Primarchs for that role, but amongst those Dorn was too stubborn and Sanguinius too ephemeral in outlook, while Guilliman had by that point already become as much ruler and administrator as he was a general.

Horus was the all-conquering warlord, a more charismatic leader than Guilliman, whose mastery of warfare was perhaps more intuitive, more instinctive than Guilliman's. In the subtlest of ways, I imagine that he treats war as art, while it is more akin to science to Guilliman. In that regard, Horus is perhaps more suited not only to planning the remainder of the Great Crusade, but leading it - being the figure who represents it in the Emperor's stead.

For the Ultramarines in battle, knowledge is key - by knowing the situation as fully as possible, an Ultramarine can best assess the required course of action. That seems very much to be the fundamental heart of the Codex Astartes: the transition between the theory of the situation and the practicality of necessary action. Where any one Astartes is equipped with a mind and senses that can take in and process the vital information of battlefield circumstances better and faster than most humans, even the greatest of them still cannot match a Primarch's mind in this regard. The Codex Astartes exists to turn the Primarch's wealth of strategic theory and tactical philosophy into a tool that an Ultramarine can draw upon in battle (an eidetic, or near-eidetic memory is commonplace amongst Astartes, and thus learning vast portions of the Codex Astartes is entirely possible), giving them some measure of the advantage of their Primarch's insight without requiring his presence.

...as we've taken up the great Codex debate, perhaps we should recall that the oldest copy is the apocryphon of skaros, if memory serves; and it dates from a time after the Heresy...thus, what we have in m40 cannot definitively be traced to the primarch's hand per se. It could be demonic forgery...

Oh, and it seems to me from sources that the Lion was the best of their military commanders; but Horus knows how people tick, and the Lion doesn't.