Okay, here is my first stab at trying to balance the diplomatic action percentages for the Galaxies rules.
Offering a Treaty: Treaty Chance - Tension = % chance must be greater than 0 to offer.
Signing a Treaty: Treaty Chance - Tension must be greater than 0 to sign, or it becomes the % chance of acceptance for a NPE.
Breaking a Treaty: Breaking Chance + Tension = % chance of breaking the treaty
Declaring War: Declaration Chance + Tension = % chance of declaring war
There's some math hidden in the table, but after trying to figure out the best way to describe it I decided that just putting the chances into a table is probably going to be the cleanest way to communicate the chances. Then you just have Tension as the final modifier, with Tension reducing offering/signing chances, or increasing breaking/declaring chances. I think that is pretty logical, right?
For example, let's say that our buddies the Krogan ran into the Narn
They have a starting Tension of 25.
At this level, they could conceivably sign all the way up to an Alliance, although that is a bit precarious given their level of relations. That and if they are a NPE then you're going to get the treaty delay involved that is going to cause some issues for them, but I digress.
Let's say that the Krogans and Narn find a lot in common and do sign all the way to a Mutual Defense pact. Great!
It's been 10 turns, and their relations have soured to 37 Tension. The Krogans no longer believe that this Mutual Defense pact is worth keeping. They have a breaking chance of -10% + 37% (Tension) = 27% chance. They make an attempt and roll "97" -- big failure!
Now, this is where having some of those diplomatic modifiers from the empire/government traits would come in handy. A +20% here or there would have really increased the chances.
On the next turn, the Krogans spend 12 EP to use 12 Diplomatic Intel to get a +12% to their breaking attempt. This increases the chance to 39%. They roll a "69". Another failure of diplomacy. Sigh.
And declaring war at this point is almost impossible due to the starting chance being -80%. You'd have to be an insanely aggressive species or totalitarian government to pull off something like that. Even then, Tensions would need to be high. This demonstrates that it becomes very hard to declare war against someone that you have signed multiple high level treaties with, and you're going to need to be able to break those treaties before you can declare a war.
Luckily for the expansionist Krogans, they just made contact with a new species!
Greetings and various apropos felicitations! We are Pkunk.
The starting Tension is 97 (oh yes!). The Krogans can sense their destiny is at hand!
Empires start in a state of Non-Intercourse, and at that level the base declaration chance is 0%. That gives the Krogans a
97% chance of declaring war on the Pkunk! Oh my!
At this level of Tension the Krogans wouldn't even consider signing a Non-Aggression treaty (I may need to bump that treaty up a bit in chance, as right now it is a bit low). They hate the Pkunk far too much for that. The only real prospect is war!
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I may need to adjust some of the treaties upwards a bit to make them more consistent with the NPE rules. For example, Non-Aggression may need to jump to 80-90%, to make it easier to sign for empires that aren't particularly peaceful. Trade could also move upwards. In 1E, the NPEs at 50 Relationship had about 90 for Non-Aggression, 80 for Trade, 60 for Naval (the Military equivalent), 40 for Mutual Defense, and 30 for Alliance. Based on that, I could definitely see moving Military, Trade, and Non-Aggression up because Tension is going to make it hard enough to sign any of those treaties with a NPE. And giving players with low Tension free reign to enter into treaties is probably fair enough.
The alien traits are going to give a +-20% swing to most of these, or at most +-40% if you have two modifiers that happen to stack (extreme cases!). A Xenophile would get +20% to Treaty Chance while a Xenophobe would get a -20% to Treaty Chance, for example. That adjusts the window in which the empire would be able to sign treaties. Similarly, an Aggressive empire would have a +20% chance declaring war and a -20% chance to signing armistices, while a Pacifist would have a -20% chance to declaring war and a +20% to signing armistices.
Then you have Charismatic, which makes it harder to break or declare with that empire. This works good for the Asari and maybe even the Federation.
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Armistices are something I'm still working on figuring out. It's likely that two empires that are at war are going to have a Tension around 100. I'm not sure we want Tension to increase based on wartime activities (destruction of systems, bombardment), but it makes sense that an escalating border war could spiral Tension to maximum. I think we'd have to integrate the "war exhaustion" rules from the 1E NPE rules so that as you lose systems and colonies that you'd gain a bonus to your armistice chance. I think that was something like +2% per turn, and then you gained from friendly military losses and friendly systems conquered or lost from enemy military losses and enemy systems conquered. That way you had a bit of a see-saw effect. But that was a lot of extra tracking, and I'm not sure we really want to deal with that in the basic rules, but there needs to be some way to determine if a NPE is going to be willing to accept peace.
I think the bigger concern there is that Tension could just continue to build to maximum throughout the conflict and that you would need to have some outlet to allow two empires to reach a point that they are willing to accept peace.
In the simplest implementation, it might be a base chance of 100 - Tension, +2% per turn, -1% per point of construction cost of enemy units destroyed, +1% per point of construction cost of friendly units destroyed. Leave conquest of systems out of the equation completely.
Example: We have been at war for 10 turns. Tension is at 100. During that times I've destroyed 113 points of enemy units, but lost 70 points of friendly units. My armistice chance would be (100 - 100) + 20 + 70 - 113 = -23%. The war is going well for me, and I have no interest in peace.
Now, if the tables were turned, the chance would flip to (100 - 100) + 20 + 113 - 70 = 63% armistice chance. That means if I'm a NPE I'd have a nearly 2/3 chance of accepting any armistice offered to me.
This does illustrate that the modifiers are a bit one-sided, and it might be better to have a fixed armistice chance (separate from Tension) and have it tick up over time and only really be concerned with the comparable losses. I don't think we can get away with more than a +2% per turn, because at 5% per turn you end up with conflicts timing out pretty fast. Within a year you'd be at 60%, which is just too fast for most wars. The 2% puts it at 24% per turn, ignoring any other modifiers.
The biggest problem with checking losses against each other is that in a lopsided war, the winning side is unlikely to ever consider peace unless the war somehow is drawn out long enough to counter their gains. It's arguable if that is a bad thing, however, or just the system working as designed. It's definitely something we need to discuss and consider. It's less of an issue for players, because we can just handwave for them that they can offer and sign armistices at any time regardless of what is going on. But for NPEs it is more of a concern.
However, that being said, I would prefer if there was consistency there, too, so that the armistice chance modifiers from traits would affect both player and non-player empires equally. Given that they are tied to traits that already affect declaration chance they'll at least still have a chance, but I really like having consistent rules.