It's important to remember that in STW/MTW the AI can see the moves you're going to make and can react to them accordingly. This is why it will leave an emissary in a province for years on end and then suddenly pull him out when you decide to target him with an assassin. It's also why the AI often inexplicably moves in reinforcements to a province on the very turn you try to invade it. In short: The AI cheats.
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