I think it's true in ETW (though I don't think it worked like this in previous games) that if you besiege a city, when the required number of turns have passed and the city can't hold out anymore, the besieged army sallies forth to fight you. When they lose, you DON'T automatically get the town. The regular army units leave the city but come your next turn, you're still besieging the city and only the armed citizenry are left. If that is true for ETW, then you could follow Marquis of Roland's advice without risking the actual taking of both regional capitals.

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