FreeOrion

Something else to consider, particularly with war declarations:

in Civ IV, if you had military units (except stealth units) located within another player's borders and declared war, your units were automatically moved outside of your opponent's borders. Should a similar system be implemented in FO?

I would say they should not. I would just make empires Very reluctant to give Open Borders... this is Open Borders for Military Units after all. (If there are 'Trade Agreements' they should not require Open Borders).

I think it Might be a good idea if there was a "Civilian Open Borders" ie Supply lines can go through, and possibly Colony/Outpost/Scout Ships, but no Military Ships

There is the issue of what to do if you have your units in someone else's territory and they wish to go from Open to Close Borders.
I think the Solution is
If in a state of "Closed Borders" and you are in their territory (whether because you entered an unscouted system or they just closed borders).... You have the option to
1. Declare War
2. have your ships auto-move to the nearest location in your Supply network.

Important to this question is the definition of "borders", which i haven't really tried to define, expect to say at minimum it includes the systems where you have outposts or colonies. If an empire's territory doesn't project or extend beyond the systems they hold, then there may be less reason for one empire's ships to cross another's borders.

But the root of the problem is the fact that it is very hard to objectively distinguish between (for AIs) large fleets crossing through territory to fight another foe, and a large fleet preparing to camp on your doorstep and then declare war.

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Well I'd go with the smaller definition of "territory"... my territory is systems where I have a colony and you don't.


Well, one thing the AI needs is a player 'Threat assessment' If the player is a potential Threat (would probably gain from a war), then the AI should either
1. Not offer Open Borders/Close Borders.
2. Only give Open Borders after it moves a Dreadnaught fleet directly above your HW. (as Insurance)

"Open Borders" would be practically an Alliance (Which is Why I think that it would be good to change it to 'Civilian Open Borders' that carries supply but not Military Ships)

Then you can have a "Fleet Passage" as a one time deal (ie I want to move Fleet A to location X, can it move through your territory while doing so?) [Perhaps you could offer Collateral, something that you would get back if you didn't veer off course]

So I'd change it to

Unification
Permanent Alliance
automatic sharing of all vision, intelligence, and tech.
Collective Victory
All have same Diplomatic Agreements (War/Open Borders) with third Parties
[has costs to establish... Trade investment over time?]

Alliance
like "Open Borders" but includes:
If someone declares war on you, war is automatically declared on all allies as well.
sharing of external vision, intelligence on mutual enemies
All Ships have access to each other's territory without specific "Ship Passage"
Peace must be made with all Allies.

Open Borders (Civilian and Supply)
Ships refueled and resupplied
Shared supply distribution network (not shared supplies)
Military Ships/Planets do not blockade supply
non-Military Ships have access to each other's territory without specific "Ship Passage" [violation->retreat or War]


Neutrality (default)
No Access to the other's Territory without "Ship Passage" [violation->retreat or War]
Military Ships/Planets do not blockade supply


Closed Borders
Military Ships/Planets Blockade Supply (Unless friendly planet present)
No Access to the other's Territory without "Ship Passage" [violation->retreat or War]


War

ships & planets cannot peacefully coexist in the same place.

Total War

war to the death, no diplomacy, no surrender.
might only be used for monsters, precursors, or other pre-scripted foes