Two armies attacking the same province

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Yes, there are some things that can prevent movement of one of the armies by removing the commander. Assassinations, random events, remote summons, "ritual artillery," horror attacks, transportation spells, HP loss from disease/etc, and weird spells like winged monkeys all go before army movement, and there is a separate battle phase to resolve anything that happens from this sort of thing. If one of these should remove a commander either by death or magical relocation, all the troops he is leading will be transferred to the province garrison and will not carry out whatever marching orders you gave them that turn. This can be a major disaster.