I find it fascinating how much reputation and emotions carry over between games if you play with the same group regularly, and also at the house rules that develop.
For example, with my brothers and a few of their friends it is entirely expected that you will get stabbed in the back in a game if it helps the other guy win. There is a lot of guarding-our-flanks with that crowd. It's totally normal and expected, however some people have more of a reputation for it than others, which can lead to some players being unable to make any pacts without excessive bribery.
On the other hand, my current social peers are paragons of morality. If someone says something regarding the game, no matter how casually, they will be bound do that for a reasonable duration. Breaking this code creates a reputation which will carry over for the next few games and you'll find very few people willing to ally with you without stringent conditions.
Another odd example is playing multiplayer Magic: the Gathering. Throughout our time there have been a few decks that may not win often or quickly, but when they get going they become unstoppable, kill everyone in a single turn, or create a global effect that makes the game "unfun" for the other players such as destroying all their creatures and lands. Some of them have gotten such a reputation that certain players have only to play a certain spell on their first turn and the rest of the table will immediately focus on killing that one person first.
> For example, with my brothers and a few of their friends it is entirely expected that you will get stabbed in the back in a game if it helps the other guy win.
My fiancée will always put the dagger in my back in games. That's love, I guess.
Junta is good. Alas, I never played, only watched people play it on a Diplomacy convention. Too bad it takes nearly as long as Diplomacy.
If you want to lose your friends more quickly, you should try Kuhhandel (http://www.boardgamegeek.com/boardgame/1117). That's poor evil `attacking', bluffing and second-guessing. (Unfortunately alliance are not common, so you do not get to break your word and stab your friends. But still a good game. :)
(Yes, I've won several games of Diplomacy. The bitterness can last a while sometimes.)