Note: Splitting off from WTV's master list as a separate entry, for the Era 4 tag to be added. I strongly advocate for this feature being present in Era 4.
Each sovereign should have a different color in each players' view:
1. White = the player's sovereign
2. Green = "friendly" sovereigns
2. Yellow = "neutral" sovereigns
3. Red = "enemy" sovereigns
DEFINITIONS are as follows:
Friendly = the player has never attacked this sovereign before. There exist functions to track this: see how the social order penalty is applied to larger empires attacking smaller ones. This is the "default" state.
Neutral = the player has attacked, or been attacked by this sovereign in the PAST, but they are not currently at "war" (see below)
Enemy = both sovereigns are currently at "war" with each other.
There should exist a button at every other player's capital, under the "Send Message" button: "Declare War" and "Negotiate Peace". Declaring war sets the state of that sovereign to "enemy", with the corresponding color change. Negotiating peace sets the state to neutral (yellow), as they were previously at war. Both of these options should spawn an automated message above the targeted player's capital: "Empire X has declared war", or "Empire Y wants to negotiate peace". Negotiating peace will require the other party to also click on the appropriate button on their rival's capital, to prevent quickly negotiating peace if the war is going badly for the aggressor.
Attacking another player's worlds without first declaring war will not be allowed. Attacking fleets that are orbiting an owned or neutral world will still be allowed, however.
Declaring war will therefore require the discovery of the target player's capital first.
The advantages of this new system are twofold:
1. It prevents attacks by "troll" empires which have been common in the past - creating a new empire and attacking all the undefended worlds in sight before abdicating.
2. It prevents newbies from unknowingly attacking a vastly more powerful empire without knowledge of their real strength (for example if they see a few dozen worlds are undefended).
In the future: an "allied" state can be introduced, but for now the 3 different diplomatic states should be sufficient. These changes would make the game clearer to newbies rather than representing every other player in red - as that color in games has always traditionally implied "this is an ENEMY - attack them!"
Please comment if I haven't clarified the mechanics of this properly (or any edge cases that I haven't addressed).
EDIT: If different fleet icon colors are introduced, changes in diplomatic state should update these accordingly.