Introduction to the fourth entrance: The fourth "Shape Chapter" discusses that when using troops to fight, we must first create conditions for ourselves not to be defeated by the enemy, so as to wait for the opportunity when the enemy can be defeated by us, so that we can "be in an invincible position" ". Sun Wu believed that the outcome of a war is determined by the strength of both the enemy and ourselves. If you want to defeat the enemy, you must have an absolute advantage in the balance of strength and create a rapid and irresistible trend. In addition, I must wait for a favorable opportunity for the enemy to be defeated by me, and be good at seizing the enemy's weaknesses, so that I can defeat the enemy easily. Sun Wu believed that to win in battle, one must be good at dealing with offense and defense issues. If you don't have enough troops, defend; if you have more troops, attack. When defending, you must hide yourself very tightly, and when attacking, you must catch the enemy by surprise. In this way, the goal of "self-preservation and complete victory" can be achieved.