I get the wars confused.
As for Dante, I always found that an odd situation - NOBODY orders around a chapter except for the chapter master, and yet I would very well imagine a space marine chapter master being very easily able to exert authority over guard forces. However, it still seems that:
1: Despite Dante commanding a force nobody else can command
2: the other forces probably following his orders, too
He wasn't in overall command.
Imperial Hierarchy is utter baffling (and illogical).
All of this has happened before.