He mentioned earlier that the point of starting the war was to get most of the soldiers sent to the frontlines, leaving the princess and her castle with far less protection.
My beef in MC thinking is, he is aware of chances there is someone/a group backing the demon, or the princess, and he think by putting the war first, it will make them sit down with pop corn watching lord of the ring trilogy.
There is no way that is the case, more over with the demon is aware of the presence of someone who capable of killing her in fair duel, anyone on this situation will be more cautious and not reckless.