Why ally with someone?

[GoodGimp]

Is there any point to forming an alliance in the game vs a peace treaty, other than winning the game if you have an alliance with all civs?

It says in the either the Great Library or the Manual that alliances allow your units to ignore allied units zone of control as well as automatically sharing maps every turn, but this obviously isn't the case... With that being so, is there any reason to ally with someone?

[LandBuilder]

Good question. No one ever wants to form an alliance with me and when I agree to their request, a couple of turns later, they want me to declare war on a country I am at peace with. I think the main reason is that you can run military units through their country and they won't ask you to leave and it doesn't count as negative diplomatic points against you.

[Maosung]

Alliances can be both helpful and a pain. My advice is to use them sparingly...and for sinister purposes only. In an alliance, your partner won't immediately attack you - in theory, access to his land is OK, and vice versa. As such, you can discretely land settlers and other units onto his land mass - ideally in places where he's not yet strong. Build a couple of strategically placed cities, fortify them over time, and towards the end of the game you'll have key bases of operation to move against...well...your age-old ally (or others). His only recourse while in the alliance is to surround your units so that they can't move (which requires lots of his units) and/or expel your units. That's why try to stay 'below the radar' at first. Eventually, he may break the alliance and attack you, but (hopefully) by that time, you will be able to fend him off. At the least, any war between you is being fought on HIS territory and not yours.

In an alliance you can also ask him to attack a common enemy. Truthfully, in general, they don't make much actual effort but it does send a wave of concern through the common enemy and can make said enemy more receptive to a peace treaty (if you need it).

Hope that helps.