Motoharu: "Great! Now all we need to do is to steal the Nagato castle while Otomo's army is away."
Takakage: "That has been arranged. Foolish Otomo left almost nobody defending the castle."
Motoharu: "Nice! But we still had to take care of Otomo's army on the field."



Note: Tried two approaches to this:

(A) Take Suo, then Nagato (as depicted in the walkthrough). The Otomo army remains on the field, and had to be killed off by the Nagato army.

(B) Take Nagato, then Suo. The Otomo army vanishes.

While (B) seems to be an efficient solution, (A) turns out to be better because (1) Defeating Otomo's remaining army creates "military crackdown", which enables us to leave no garrison in Nagato - this army will be needed elsewhere as the story will later reveal; (2) One extra fight brings more experience to soldiers; (3) Suo can be taken one turn earlier for a little extra profit.