Open Rebellion: May take place on any inhabited planet that is being affected by enemy culture once allegiance drops to 30% or less. When this occurs, the planet is flipped to neutral (unowned) and an uprising occurs with a fleet of ships spawning at the planet.

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1) Truce Amongst Rogues makes this event weird.  You lose the planet and get a borderline gigantic CPU defender fleet.  Then you reclaim it with ease.

2) This event is bugged and will occur over and over on certain map setups/game states.  Combined with the facts that TAR makes the fleet an ally, and the rebels don't count towards fleet cap, you can end up with a chokehold planet with like a 6,000 point fleet defending it.

2b) The event triggers even if only one faction's culture lines are touching it.  In my experiment, I lost a planet to it, and the planet was only accessibly via wormhole - no enemy culture anywhere near the planet.  It had the anarchist society "bonus" so max ALG was ~25% - that seems to be the real trigger.