Ha, my favorite topic. I think aliances are a core part of the game, especially on certain conditions/maps/game options.
Better call them "truces", or "agreements" though; people don't like the word alliance as it implies ganging up on someone.
Now, first HUGE problem is that not all players even look at the chat. They do themselves a great disservice with this. LOOK AT THE CHAT, it SAVES LIVES!
Typically,
agreements come into play the most in conditions where owning continents matters. So you'd find them less in Escalating games, and more in no-cards games. In some formats they'd be completely silly, like Assassin. And in some cases they're almost implied: like in a 3 player situation with 1 playing the dominating force, it just simply doesn't make sense for two weaker players to weaken each other even further. In this case you need not even talk about "this big Alliance", merely stating something like "Ok Blue I'm going to leave you alone for a while because if we keep banging on each other then this will all be over in a couple turns and Red will walk the board.."
I greatly prefer wording it in that way in a positive 'matter of fact' kind of statement, often citing a quick and completely obvious analysis of what's going on. I prefer this to an 'asking' format like "Blue, can we do an alliance against Red?" I think that sounds bad, and is not very specific.
Personally I also like people who USE the chat, even if 'just' for chat. I don't like players who are quiet all the time and then suddenly announce: "Red, 3 turn truce in North Africa/Brazil?".
Then, there's a lot MORE than just truces, often if you just ask, people will MOVE out of your way! Like:
"Blue, I how about defening S. America from Brazil and not North Africa.. My only hope in this game is getting Africa so I'm gonna go for it next turn.. I'd like to do it without wasting your and my armies unnecessarily and killing both our chances.. It's your call of course."
Or, on the Europe map, something like "Pink, I can see you fancy Scandinavia and you're welcome to it, don't put any guys in Denmark and I'll move out of the whole area next turn." (this is especially useful in Escalating games where some fairly newbie people still desperately go for continents, as if a 3 or 4 army bonus matters in the slightest when 15-20 army bonuses are flying around by turn 7 or so.

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Alex..? Alexander the Great, my Commander, what's wrong? Did someone leave you negative feedback again?