[Taqwus]

Convince it that it's about to die? In particular, if you've reduced one to a single system, its fleet is gone, and all or most of its ships are being orbited by your warships, your odds should be much better then if you were merely at the gate. If your ships are cloaked, it probably helps to uncloak them just to convince it that they're actually there.

OTOH, it's possible to get an AI to surrender without a single fight. In that no-warp-point game I tried, some of the AIs surrendered right after my fleet entered and made the demand -- they couldn't view my ship designs (no fights, no sensor technology), but it could tell the size of my ships and fleet, and it may also have used relative score as a criteria (e.g. if it's never left its system, and it sees a fleet belonging to a player with ~7 million points, then obviously it's going to lose... so surrender is a smart choice.)

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-- The thing that goes bump in the night