• Summary / Title: Privateer fleets don't have the option to invade since they can't siege other factions' systems.
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• Description: The option to invade a system is unavailable for privateer fleets.  The tooltip says invasion is unavaiable because the system is not under siege.  There is no way to direct the privateer fleet to put the system under siege.  I'm playing without the DLC, in case that makes a difference.
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• Steps to Reproduce: Send a privateer fleet to another faction's system.  Note that invasion is unavailable.  Interestingly enough, if you put the system under siege with a non-privateer fleet, the privateer fleet can then invade.
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• Reproducibility: 5/5
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• Expected Behavior: There should be a way to direct privateers to put a system under siege.
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• Version: V1.2.6 S5 GOLD-Public
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• Attachments: 
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I can confirm that privateers no longer siege a system (which were able to before).

QuickSave.sav

Someone wrote about privateers not beeing able to siege, because the faction does have non aggression pact contract with pirates. The reason privateers can't siege this case.

Groo wrote:

Someone wrote about privateers not beeing able to siege, because the faction does have non aggression pact contract with pirates. The reason privateers can't siege this case.

No, i think its a bug. In this screenshot i have mark the system with no problem (not possible if the AI had non aggression pact with the pirates) but invade is not possible :/

Devs work on it: Vaulter Piracy Invasion ?