i loved the book, first real introduction to Iron Warriors for me. I've started the Iron Warrior's Omnibus and I'm a bit confused, maybe someone can clear it up for me. Kroger and Forrix are Perturabo's his #3 and #1 guys in Angel's right? then why in the omnibus do they feel like they were demoted or passed over by Warsmith? (I'm only half way through Storm of Iron so please no spoilers) if it's explained in Storm just tell me to ask back once I finished with that story.
My personal theory was that after the Iron Cage incident, when the Iron Warriors finally retreated to the Eye, their Legion broke up into various warbands. At that time, the three of them (Warsmith, Forix, Kroger) formed their own warband. Hence why they are together during "Storm of Iron", thousands of years later.
Assuming this did happen, I don't think it's a matter of demotion or being passed over. Kroger was by no means a prodigy. With the destruction of his Battalion, Forrix might have already decided to be more of an advisor/champion than a commander. As such, the Warsmith may have naturally fallen into the position of command between the three.
I took the reason Perturabo rebelled was due to being tired of being taken for granted by his brothers and father, they mention how the one planet had statues/paintings of Ultramarines and Imperial Fists everywhere but only one painting with Iron Warriors.
That's one reason, yes, but there are other factors as well. A central one is that Perturabo was first and foremost a builder of things, and thus despised siege warfare - which left him destroying things over and over. In the pursuit of the style of war he hated the most, he lost proportionately more of his sons than the other primarchs did... and he lost them in ways where they couldn't even claim glory. It all added up, basically.