Breaking a peace treaty that is approved by the others, would signal your disregard for them. This should, and would, rightfully move you up the list of threats.
So you're saying that you believe DC should be able to not only force the target player into a peace treaty, but also force every other player into backing/approving that treaty, without providing benefits to any of them? I'm been talking about persuading people with diplomacy and negotiation, you're talking about magically coercing them to your will. Methinks you are treating "diplomacy" more like something I would consider "conquest".
Equally, this is redundant in multiplayer. A human will be happy to lose a rival if that rival doesn't help them, but wary of the military rival whether they break peace treaties or just happen to be very big. The notion only exists as a metagame concept. Give an in-game entity reasons for their actions and this ceases to be purely a metagame consideration.
Last point: Now you've made diplomacy into a tool for losers. If you're losing, be diplomatic so you can take them with you!.. what? No. Make diplomacy a tool for winners. Use diplomacy to beat opponents, not just beg them into leaving you alone. Giving them a forced peace treaty to break doesn'tmake your imminent loss any less pathetic.
As for having gameplay "forced upon you?", I see this as no different as someone rushing you. That's forcing a military game on someone.
Certainly, it's forcing a defence to be created or you suffer the loss of a particular type of resource - cities, territory and the like. This doesn't force a "military" response, however, so long as the game mechanics are sound. Imperialists can construct city fortifications - early units have little hope of penetrating even basic city walls or dealing with arrow towers; Spellcasters can summon up some defences, or blast them to ashes; Adventurers can allow their fledgling heroes to cut their teeth on the invaders.... Even here, no action has been forced - just because you put someone on the defensive does not mean you control their actions.
How might a diplomatic player deal with a rush without forcing the player to do something?
Well, there's the already mentioned diplomatic approach of buying the leader off by offering them tribute - persuading them to stop. Any branch of the five game mechanics can attempt this, and every branch has something to offer. Diplomats have an easier time however.
Secondly, there's the potential for bribing a unit, sending an Ageny to them before they reach you - this isn't automatic, a unit might kill the agent, keep the bribe (giving it to the controlling player) and stay loyal, or turn the agent away.
First of all: to switch sides. This wouldn't work against a large force, but it has a good chance when dealing with a small one, where you've basically removed a significant fraction of the threat.
More easily, the diplomat can send an agent to bribe the stack to leave. These bribed units will "grey-out" and auto-move back to their home city, unless they're attacked or blocked on the way.
Even more easily, the agent can coerce them to simply grey out and stay still for a few turns or until attacked/a non-grey unit joins them. This gives time to sue for peace or request allies to help you, and an attacking player may agree, just so he can get his military back from being paralysed by red-tape and corruption.
The more experienced, value and high morale a unit has, the harder they are to bribe. Heroes are hardest of all (but shouldn't be able to conquer cities by themselves anyway so aren't appropriate for "rushing") and virtually never switch teams. This puts the diplomat at war into improving their diplotech to get better at bribery to keep a decent chance of paralysing and removing approaching armies... But this isn't easy, since diplomats are also supposed to have a good reason to keep them around in the first place - being more valuable alive than dead through trade and good relations.
If you're being overmilitaristic, you should expect a hard response from those who can resist, and those who can't will do what they can behind the scenes. if someone wants to win by that scenario, they should and would be able to go for it, but there would be consequences, and realistic ones (AI dogpiles would be reasonable, a human would do the same thing, and I have in games of GCII when the Torians go on a rampage)
This ignores the stated five-corner principle of the game mechanics. Over-militarisation is not an auto-win if Conquest is only one, equal, component of the game. In Civilization, there are really only two aspects of the game - Construction and Conquest. Construction allows access to resources, research, "votes" and basically everything of value; Conquest is the means by which those resources are claimed or defended and is otherwise worthless, but not balancing both aspects is costly, because Conquest can only be countered by Conquest in Civ; what do you expect from such a simplistic game? You're always looking to see if they're conquerable, because it's one of the only actions you can perform.
With Elemental's proposed system, if someone is hyper-militant and has armies bigger than every enemy combined, then they're simply focussed on Conquest over the other four paths. Specialisation has its advantages, but in general with perfect game balance they won't be better off than an equally advanced player dedicated to another path, or much better off than an equally advanced player who generalises across the five.
Who can resist? Everyone can resist, but they do it in different ways.
I don't think war should bleed DC, I mean, it's a bleak world, and war is probably common.
This is at odds with the idea that other leaders will support a peace treaty. Either war is common, and the people apathetic, or war is unpleasant and disliked enough to offend people about it.
Possibly another name for DC could be "Goodwill"? Goodwill comes from peace, stability, fair dealings and the like. War does not promote goodwill, indeed, probably loses you some, but agreeing to peace might.
I don't mind the AI being a bit "stupid" when it comes to Diplomatic victory. If you earn the win this way, you earn the win.
So long as there's a non-stupid diplomatic victory, I don't see a problem. I think the primary disagreement here is that I want diplomacy to be equal to the other five - not something that only exists in single player and is ignored anywhere else, because that isn't in the spirit of the design principles given.
I would prefer a diplomatic system that makes sense in both multiplayer and single player, not one that you "switch off" because it's unrealistic and end up with a completely different experience that you may as well not be playing the same game. Diplomacy has techs related to it, an entire tech tree for potentially massive investment. It should be just as worthwhile investing in for any game - single player, sixteen player, or even one versus one.
It's possible to do, it would be fun to achieve, it would make single and multiplayer better, and it would make Elemental stand head and shoulders above its competition.... why chicken out of making it happen?
To this end, what needs to be considered is:
Equality in game mechanics.
In a way, basic diplomacy is open to everyone, and this basic diplomacy is simply going onto the trade screen and saying "I want X, what do you want in exchange?". X can be anything, from 5 golds to trade sanctions to "ask Player Two to do this for me". Basic diplomatic activity is automatic, just like basic imperial activity (build cities, gain resources), basic conqueror activity (build spearguys and swordguys), basic magical activity (at minimum cantrip spells and assumed magical talents), and basic adventurer activity (low level heroes will seek you out). This is zero investment standard game mechanics available to everyone.
What does investment offer?
For Conquest, this is obvious. A conqueror has better units. A conqueror gets special units that can do things far beyond the understanding of a basic swordguy. He gets sappers, demolition experts, kung fu monks...
Is this all? No. He benefits everywhere. He pays less for his armies than other people, and he can even send his special ops in to handle situations that usually would need a hero. He can lend weight to requests in basic diplomacy thanks to his sizeable armies. He can clear out monster lairs to help out his spell research, and he can go out and conquer other peoples' cities and raid them for resources.
Conquest has a win condition - By... uh.... conquest, that can be resisted by not being conquered.
For Adventure, this is obvious too. Heroes get better, get more potential to improve, open up new quests to go on....
And gets? The same thing - The base-line of everything improves. Heroes get you unique stuff to offer in trade, make AI guys to like you, a hero leading an army of basic swordguys can actually be a dangerous conquering force, dungeons contain all kinds of resources, cash, military units, spells.... And a hero can always retire and become the governor of a town to make it better in all sorts of ways.
Adventure has a win condition by ganking the Forge of Whatever. Players can stop you here too, whether they're competing for the victory type same as you are, or not.
Magic has it easy, magic does a little of everything by default anyway through its spells and mage-type units and summoned units, I barely even need to mention it.
Magic has a win condition, but it's hard to disrupt it without seriously interfering with the mage's activities. As a result, magic's win condition is late game and ties up a lot of resources.
Notably for both Adventure and Magic, their win conditions are auto-wins against all players at once, so their activities are defended against by all players at once. Conquest has a slight advantage and disadvantage both that it's win condition is single-target specialised, so it's pitting its resources against each player directly, and must divide its resources if it wants to do more.
Imperialism has it a little harder - Imperialism lets you build up cities for better resource management, lets you construct buildings that help each individual corner - libraries for research, training centres for faster military unit production, better mines for superior unit equipment, and lots of gold for bribing people in basic diplomacy.
Imperialism lacks a win condition, and city fortification hasn't been defined. Imperialism also has the problem of being purely defensive. This means imperialism is not a game winning strategy, and doesn't have any way of dealing with other branches of the tree without investing in the other aspects - this is a design flaw, but not one this thread is concerned with.
Finally, the presently discussed:
Diplomacy - Hasn't got any real benefits in most games - It's better to use your military to conquer that guy and thusly get his stuff directly than it is to leave him in charge and barter for it. So diplomacy is only used as a delaying tactic or for basic trading. In multiplayer, there's no benefit to being "nice" like this, and people aren't programmed to be nice for the sake of it, so there's no diplomacy.
Now diplomacy can have great effects because of DC. This can reduce upkeep for all types of resource, make trading for resources easier, stealing them from your enemies with spies and create your own military by stealing other people's. People do get tangible benefits to being nice, because being nice generates DC, and players can offer DC to each other in exchange for them "being nice". Diplomacy can be used as more than a glorified "I'm not conquering you yet" holding pattern.
In fact, Diplomacy, like Conquest, should have a one-target at a time victory condition, not a universal win condition, if only to make a pleasant change over the other sweeping victories in the other two branches with current win conditions. Diplomacy is between you and me, after all.
Even in a 1 vs. 1 game, a diplomat can send in their spies to attack an enemy player - as mentioned, by paralysing their military units, sending them false quest information to tie up their heroes, steal their resources, destroy their storage utilities, cripple their defences, ruin their spell research and countering their spies with your own counteragents - and without having to spend it on trade or asking for favours, they have plenty of DC to fuel these illicit attacks whilst it works on removing the other guy from power or flat out assassinating him.
Diplomacy can be equal to any other path if handled properly. If it's handled the same way it always has been in other games, with DC as just some pointless gimmick, it will remain irrelevant. It needs a mechanically viable means of harming other players, helping them, and a method of winning in its own way, with the resources associated with it.
And don't think of it as being long-winded, think of it as being thorough!
Or rambling. Rambling is a fun word.