“The art of war, then, is governed by five constant factors, to be taken into account in one’s deliberations, when seeking to determine the conditions obtaining in the field. These are: (1) The Moral Law; (2) Heaven; (3) Earth; (4) The Commander; (5) Method and Discipline.”

The first great principle of the book is to take all matters into account that may affect the outcome of a war. Moral Law is the quality of the ruler that affects the citizens’ willingness to make the sacrifices required during wartime; Heaven is weather and season; Earth is the geography of the conflict region; the Commander is the army general and their abilities; Method and Discipline are the army and its organization and strengths. Thus, bad weather or rough terrain can make a war more costly, while good leadership and discipline can help an army overcome large odds. 

“Hold out baits to entice the enemy. Feign disorder, and crush him.” 

Sun Tzu stresses that victory relies on deception. It is always vital to appear strong when weak and weak when strong, so that the enemy miscalculates, positions its forces in the wrong places, and over- or underestimates its chances. These techniques also demonstrate an army’s efficiency and adaptability, attributes that help reduce the deadly cost of warfare. 

“Hence a wise general makes a point of foraging on the enemy. One cartload of the enemy's provisions is equivalent to twenty of one's own, and likewise a single picul of his provender is equivalent to twenty from one's own store.”

Supply lines bring food and weapons from home, but this can bankrupt a nation. When on the attack, it is better to supply an army with goods taken from the invaded country.