I have a thing for 4x games but so far i have not found a game with a good AI capable of dealing with complex diplomacy.
To be more specific, i'm trying to find something similar to Civilization but with a system that would allow for complex alliances, treaties, trade agreements and so on.
For example, i would love to recreate a situation like pre Crimean War europe, with one country controlling a strategic asset (bosporus strait) that another country wants for itself, and a couple of other countries giving the first party a guarantee of neutrality and mutual assistance that would lead to a general war in case of invasion.
On top of that it would be great if yet another country would use the opportunity to declare independence (if its a colony) or settle an old score with one of the war participants, leading into even more conflict.
Is there any game out there that has a complex diplomatic model like that or even an AI that can deal with multiple treaties and react convincingly?

Hearts of Iron 2, or 3, or arsenal of democracy or something

I was quite impressed with GalCiv2.

Paradox games have pretty good diplomacy (Europa Universalis series, Hearts of Iron series, Crusader Kings, Victoria, etc). Space Empires 4 and 5 also have very interesting diplomacy models.

Seconded
As Wolfox said too, Europa Universalis
Then agan, the AI is comparatively bad...

EU3 with all 3 expansions is actually pretty solid. There are some weird AI glitches, but the other nations have gotten a lot smarter about playing for their own interests.

Yeah
God I hate it when you buy a game, and then some sweet-as expansions come out, but you can only get them digitally, and they're the same/more expensive than the game itself...
Oh yeh, and when they require the previous expansion TOO

GalCiv2.

Medieval 2 total war is what im into right now. Not much for diplomacy but it's in there.

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This. It's awesome.

The only diplomacy I get in that game is something like, and i paraphrase, "We hate you, stop doing what you are doing or we will attack". No matter what I do after then, they attack. I usually play on hard/hard, sometimes on very hard for either (but rarely both). I always tend to make an economical juggernaut though, so maybe they just always feel threatened. Fun game though, even though (or perhaps because) I always end up fighting a multi-front war, having to win heroic victory after heroic victory. I always like to play France and pretend that I am napoleon... that isn't weird is it?
As for games with good diplomacy, I honestly can't really think of any. I saw this thread a few hours ago, and have been trying to think of one in the back of my mind much of the time since. Either they are too easily duped/predicted, or they are so unpredictable as to be schizo.
I think Master of Orion 2 had decent diplomacy, but it became insanely predictable after countless play throughs. Master of Orion 3 was actually rather good, after the community patch(es) were installed, but it too became predictable once you figured out that the lizards and the aquatics always hated each other (it was almost the plot of the game, if a randomized TBS game can have a plot).
The Total War games might have had good diplomacy, but I usually did mine by knife-point. I would just assassinate and/or steamroll Empires, and as questions later. So, I am probably not the best judge there.
Sins of a Solar Empire I played only briefly. I liked it for what it was, but I really don't like RTS games. It was very good, or it would not have kept my interest for as long as it did. I honestly can't remember anything about diplomacy, but they may have added some latter (especially with this new Diplomacy DLC).
I just can't think of ANY games where you make treaties over resources, like was mentioned int he OP. There are games with research treaties, trade treaties, cease fires, alliances, and such, but nothing claiming a certain plot of real-estate (that isn't a planet or country or something). Sounds like an awesome idea, it must have been done before... but I also must have missed it when it happened.

In those two paragraphs you explained it better than i could, i just can't find anything out there where the AI would give a strategic asset some importance, you would think that a chokepoint or a strait or even a mountain range/pass would be valuable but nope.
And as for the alliances, you can forget about it, most games revolve around the "if you get too powerful everyone is against you" type which i find kind of dumb. And the one thing that cracks me up is that every game treats "peace" as "guys, stop shooting". I want to see someone implement de-militarized buffer zones, territorial grants, dis-armament and so on.
I have GalCiv 2 and that is basically as good as it gets for grand strategy but is still not good enough. It doesn't allow to coordinate war planning with your allies. ("Yo, you bomb those planets while i conduct guerre-de-course").

You probably need human players to make this work well. At least until game AI gets more sophisticated.