Probability of winning or losing in combat game with health factors for each unit.

In "Civilization", each unit has attack, defense, and health factors. For instance, an armoured unit has an attack factor of 24 and the unit defending has a defense factor of 14. The attacker has a health factor of 4 and the Defender has an attack factor 2. The Basic Odds are easy: 24/14. Losing to a Defender with a health factor of 1 is easy: (14/24)^4. But how does one calculate the Odds if the Defender has more than 1 health factor? (and did I make any mistakes above?)