Resolving Diplomatic actions in a RISK-style board game
A question to consider is whether diplomacy should even be something that is directly handled by the game mechanics, or something that happens purely through negotiation between players with game mechanics that encourage negotiation.

A great example of this is, of course, Diplomacy. There are no mechanics directly related to diplomacy (nothing that says you're in a treaty, or that diplomacy was successful or not), but the fact that moves are submitted simultaneously, and that you generally need support from multiple armies in order to take territory, means that you need to ally with other players in order to get ahead, and you need to place a certain amount of trust in them that they will do what they say; and those facts means that there are complex, shifting alliances throughtout the game, backstabbing, and negotiated two or three way wins at the end.

Another good example of this style of encouraging diplomacy and backstabbing is A Game of Thrones.

Beyond simultaneous movement mechanics, another way to encourage diplomacy without adding it explicitly to the game it through trading. If people are able to trade resources at some point in their turn, like in Settlers of Catan, then they may be able to negotiate a non-aggression pact and/or trade some resources, giving the players who negotiate such a pact an advantage over the other players in the game.

I would be wary of making diplomacy be something you roll for. It's very easy to add too much rolling for action resolution to the game. People will wonder when playing "why can't I just negotiate directly, why do I have to roll randomly to negotiate, even when what I'm proposing is directly to the advantage of both players involved?" Try to make the diplomatic aspect more of an emergent property of your game, which comes about not because of explicit rules, but because it's the most sensible way to play the game, and I think your players will enjoy it much more.