In the given situation, I would say you make your decision based on the assessed skill level of the opponent, as has been said. However, as a diplomat, you need to play every possible angle. Every move, by every player, every turn, has an effect on your situation. You need to figure out the optimal move for each player and then consider whether that corresponds to the optimal move for each set of players, i.e. alliances. When the lists of moves match, you know that person's moves. When they don't, you need to figure out which set they will play. This is largely determined through diplomacy and gauging how each player feels about all the others. Usually, there is one or two players who will be willing to give you a good amount of intelligence on the other players. That information is a good start point for figuring out what is going on with the other player's moves. Once you adjust that information for self-serving deceit and weigh it against the bits of info you get from others, you have an even better knowledge of what's going on and can make the determination of whether your opponent is defending with move list A or move list B.