The exact way on how combat-results are calculated is secret, or else you could predict the outcome. Some of the variables that will be taken into account are:

Number of units, Unit-types, Unit/effectiveness, Terrain and Quality of the units (attack – defense values) and of course your nation’s morale.

Attack and defense values are not related to each other. An INF unit will always be better at defending then attacking. Also the attacking and defense values for different unit-types are not related to other unit-types. An attack value of 10 for Infantry is not the same as an attack-value of 10 for Artillery