Most games I've played with political/ally aspects haven't ever done very well. Most of the politics in games are nothing more than a delaying action of when the AI is going to backstab you and attack. Or they just do rediculous things like sue for peace and give you 10000 of whatever the currency is and then the next turn or even the same turn declare war on you again. HOI has a lot of "events", but, I wouldn't say it's that political, much of the alliances in that game are set in stone anyways from the events. Like trying to get the US to side with you as Germany and attack Russia (not that you even need an alliance to attack Russia and win in that game). Not saying it can't or won't happen, but, it is definitely something that's a rarity. Galactic Civilizations II has some decent diplomacy of most all of them I've played though it is still very limited. It's more of a trading diplomacy (the one thing I find that's rather stupid in GC II is that enemies will trade with you even during a war with them lol), but, at least the alliances work in it. Oh and Spartan has another fair diplomacy system. There's over 50 diplomatic features in it and if you play conquer the map as I do you can get some decent alliances if you play it right, but, it's failure is that it won't backstab you and thus gives the player the advantage in the long run of picking their time to backstab the computer AI. Crusader kings had alliance diplomacy and that's about it. The alliances are usually weak ones, they don't last very long and it's rare the computer AI will ever come to your aid unless you have an extreme advantage and of course there's something in it for the alliance AI to take from you during the war.
For the most part political diplomacy just doesn't work in most games because the AI is lame using it. It's ok for trade agreements and short term alliances, but, when it comes to wheeling and dealing for provinces or border openings or even coming to your aid when needed they have all been pretty lame. There shouldn't even be border opening approval when people are playing against the AI because it's one of the biggest exploits players use to get close to the AI cities or capitals. I know I use it in every game that allows it. It's a pretty stupid AI that opens its borders and then allows you to set 3 full stacks of units on its capital doorsteps or any of its other towns/cities/planets and then just freely pick your time and moment to attack. Never liked that structure of open borders in games. Civilization does it, Age of Wonders does it, GalCiv does it, Spartan does it, in fact just about every game that has that open door policy. Humans cannot be trusted and thus AI's should be programmed to not trust them...ever.