Tu Mu says: “As water flows downwards, we must not pitch our camp on the lower reaches of a river, for fear the enemy should open the sluices and sweep us away in a flood. This is implied above in the words 視生處高. Chu-ko Wu-hou has remarked that ‘in river warfare we must not advance against the stream,’ which is as much as to say that our fleet must not be anchored below that of the enemy, for then they would be able to take advantage of the current and make short work of us.” There is also the danger, noted by other commentators, that the enemy may throw poison on the water to be carried down to us. Capt. Calthrop’s first version was: “Do not cross rivers in the face of the stream”—a sapient piece of advice, which made one curious to know what the correct way of crossing rivers might be. He has now improved this into: “Do not fight when the enemy is between the army and the source of the river.” So much for river warfare. 7. 絶斥澤惟亟去無留 In crossing salt-marshes, your sole concern should be to get over them quickly, without any delay. Because of the lack of fresh water, the poor quality of the herbage, and last but not least, because they are low, flat, and exposed to attack. 8. 若交軍於斥澤之中必依水草而背衆樹此處斥澤之軍也 If forced to fight in a salt-marsh, you should have water and grass near you, and get your back to a clump of trees. Li Ch‘üan remarks that the ground is less likely to be treacherous where there are trees, while Tu Yu says that they will serve to protect the rear. Capt. Calthrop, with a perfect genius for going wrong, says “in the neighbourhood of a marsh.” For 若 the _T‘ung Tien_ and _Yü Lan_ wrongly read 爲, and the latter also has 倍 instead of 背. So much for operations in salt-marshes. 9. 平陸處易而右背高前死後生此處平陸之軍也 In dry, level country, take up an easily accessible position This is doubtless the force of 易, its opposite being 險. Thus, Tu Mu explains it as 坦易平穩之處 “ground that is smooth and firm,” and therefore adapted for cavalry; Chang Yü as 坦易無坎陷之處 “level ground, free from depressions and hollows.” He adds later on that although Sun Tzŭ is discussing flat country, there will nevertheless be slight elevations and hillocks. with rising ground to your right and on your rear, The Yü Lan again reads 倍 for 背. Tu Mu quotes T‘ai Kung as saying: “An army should have a stream or a marsh on its left, and a hill or tumulus on its right.” so that the danger may be in front, and safety lie behind. Wang Hsi thinks that 後生 contradicts the saying 視生 in § 2, and therefore suspects a mistake in the text. So much for campaigning in flat country. 10. 凡此四軍之利黃帝之所以勝四帝也 These are the four useful branches of military knowledge Those, namely, concerned with (1) mountains, (2) rivers, (3) marshes, and (4) plains. Compare Napoleon’s “Military Maxims,” no. 1. which enabled the Yellow Emperor to vanquish four several sovereigns.