In this period, military theory made great progress. Antoine Henri Jomini, who served under Napoleon, wrote Napoleonic art of war as military theory in his "Summary of the Art of War". Jomini said there are few rules in the science of war. Jomini's theory gave reasons on how Napoleon won his wars by keeping the universal rule of war. Clausewitz, who served under Prussia fighting against Napoleon in war, also explained the nature of war through scientific studies on the Napoleonic wars. In his book "On War", Clausewtiz said the nature of war has laws that policy controls military operations in war. He also wrote that the mathematical model called law of number can be used to understand and how many results happen in a fight. In the debate of whether attack or defense was the stronger military action, Clausewitz supported defense because defending forces are able to use positions in a battle.