I think the UN troops shouldn't be present in all cities every time. The Apostolic Palace can check if a Nation is a Member or not, and the Progress of Vienna SHOULD do it, too. We could make an UN Embassy Building available once the UN is build. If you build it, you are a Member. And if you are a Member, the follwing happens:
- You sign open boarders with all members
- Improves Relations with all members
- When you are attacked, a selection starts for all Members and they decide if they help you (declare war on your enemy AND/OR just send UN Troops AND/OR using Embargos) or not. Refusing to help will lead to a diplomacy penalty and maybe even an autobuild pseudo-building that replaces the UN Embassy.
- If you get help from them, UN Troops spawn in your cities (can't move/attack) 2 turns after the war was declared. When the war is over, those units will disappear.
- When you want to declare war at someone, this will start an election, too. It would be best, if the AI is very unlikely to help you, except your target had done horrible things recently (last 100 rounds approx) like: Using/having biological/nuclear weapons, attacked weak nations, uses slavery/cannibalism/human sacrifice, using lots of hidden nationality units, (having Oil Wells

)...
- If a nation not in the UN is attacked, this will also start an election to help them or not. Refusing to help leads to less diplomacy penalty then above.
- If you declare war on one UN Member, you will get an extremely high diplomacy penalty that resulting in war against the UN in most cases.
It would be cool if there was more of those modern alliances that would benefit tech diffusion/research, gold, import/export... for all members.
Those could ultimately lead to peacefull overtaking of nations. For example if Germany, France and Spain would create the EU Embassy, it would be possible to get an event that will make those 3 disappear and spawn the EU Nation that has controll of all units and cities germany, spain and france had before. You as a player will ALWAYS be the new leader of those new "nations". The event should be choosable, so you can decide to stay seperate nations if you wish. If Italy decides to join the pact later, there should be an election, too. After a couple of turns with improving diplomacy, research and gold bonuses for the EU and Italy, it enables the decision to merge italy in the "EU nation".
This would work for Africa, Asia etc as well, and I don't think they should have such names like "EU" since you can start as Japan next to Mayas, Aborigenies and Zulu and still want to have an EU-like pact with those.
This will gradually reduce the number of civs in a game so it makes more sense that "you" can take over a whole solar system later.
I like this idea so much that I think it should get it's own thread to be discussed. What do you think?