My first attempt at the campaign failed because i could not split my army.
It started when I fought the dragon a number of times and reloaded the game to the point before fighting the dragon. I did not realize reloading a saved game does not give you the game you saved! Only realized after spotting all my mana had depleted, somehow it remembered how much mana i used in each battle and deducted it from the saved game meanwhile duplicating all my heros.
I ended up with three pages of heros all with duplicated equipment which was nice, but with so many, it was just ridiculous and i had to start over.
Now i always make sure to put a couple of turns after any saved game before combat, then also i always exit the game before reloading that saved game, this seems to prevent any saved game 'editing' going on.