Alliances?

You can send Gems/Gold/Items, but no troops. Though you can arrange for a "Trade" with Commanders with a degree of success.

I don't think a Supply or "Going through Allies Territory" is a bad idea at all. Though how hard it would be code would be interesting.

I however, don't feel that shared vision should be given. Because even if you're allies, sometimes you like to keep some information to yourself.