If someone attacks a territory with 1 army in it that is not in a full continent, no penalty
If the game is down to 2 players, no penalty
If you break an alliance and your alliance partner / opponent has the next turn, no penalty.
Otherwise, if a player breaks an alliance by attacking or by just breaking it on the player page, for their next turn, every dice roll vs the old alliance partner should be lowered by one. The alliance breaker would be at a significant disadvantage as every 6 turned into a 5, etc.
This would be a good deterrent to those who make and break alliances.