Risk Probabilities
Nick Berry (via Dave Dribin):
With just a small number armies on both sides, the defender has the advantage. This is because, in the result of a tie on the dice, the defender wins.
For battles where the number of attackers and defenders is the same 1v1, 2v2, 3v3, 4v4, then the defender has the mathematical advantage. However, once 5v5 has been reached, advantage swings to the attacker (because the benefit or being able to roll one extra die each round becomes more significant than losing on a tie.
