Combat is conducted by totaling one's attacking strength point total and rolling under that amount inflicting a numerical loss on the defender. Terrain can modify the attack roll. The defenders then can counterattack with their strength muliplied by 2 or 3 depending on their decision to retreat or not. Sole attacking artillery can only be counterattacked by artillery. Headquartes control supply which is critical.
Supply is used for attacks and defense and the number of points available is limited.
There are several other rules sections to cover game specifics regarding fortresses, trenches, artillery, cavalry, and mobilization. Overall, I must give my hearty recommendation for this game. Terrain, low movement allowance, and ZOC prevented me from bombarding Liege until August 4th. It does seem a little too late. Perhaps if some of the rivers were negated by rail lines then it would be faster.