The Great Crusade is in its 203rd year. The Emperor has retired to Terra to work. Day-to-day-running of the Empire is managed by his Lieutenant Malcalor, while the military forces are lead by his most beloved and most capable son and general, the Warmaster Horus.
Now a psionic information reaches the Emperor from Magnus, that Horus has rebelled on Istvaan. Big-E is angry, for this kind of message has been forbidden by him, and the message damaged the psionic defences of the Imperial Palace. So Leman Russ is send to Prospero to attack Magnus.
Unfortunately, the information received was true... So within 6 Months, a large force of all available SM-Legions is mobilized (8 Legions including the Imperial Fists, who will be late and ambushed enroute to Istvaan).
Horus was the most capable of all the Emperor's Generals...
Should the Emperor have expected any kind of a trap?
Should he have taken command himself instead of leaving command to one of the Primarchs?
Did he really expect Horus waiting unprepared for the unavoidable attack by the loyalist forces?