otherwise...
Learn to recognize your chances. The variables are off/def values, experience/health, terrain, city-size, improvements, and oh yaa the pseudo random number generator. I keep seeing references to a combat odds calculator, but I have no interest in looking it up. I can usually eyeball the deference between loosing 1/100, 1/10, 5/10. Most of my battles I'll accept 1/10 loses. Very rare circumstances will I continue a battle where I'm losing anything close to even. If you want to win 99/100 make sure the off/def values are significantly in your favor and/or there health is red.