["In order to tempt the enemy to cross after you," according to Ts’ao Kung, and also, says Chang Yu, "in order not to be impeded in your evolutions." The _T’ung Tien_ reads, "If _the enemy_ crosses a river," etc. But in view of the next sentence, this is almost certainly an interpolation.] 4. When an invading force crosses a river in its onward march, do not advance to meet it in mid-stream. It will be best to let half the army get across, and then deliver your attack. [Li Ch’uan alludes to the great victory won by Han Hsin over Lung Chu at the Wei River. Turning to the _Ch’ien Han Shu_, ch. 34, fol. 6 verso, we find the battle described as follows: "The two armies were drawn up on opposite sides of the river. In the night, Han Hsin ordered his men to take some ten thousand sacks filled with sand and construct a dam higher up. Then, leading half his army across, he attacked Lung Chu; but after a time, pretending to have failed in his attempt, he hastily withdrew to the other bank. Lung Chu was much elated by this unlooked-for success, and exclaiming: "I felt sure that Han Hsin was really a coward!" he pursued him and began crossing the river in his turn. Han Hsin now sent a party to cut open the sandbags, thus releasing a great volume of water, which swept down and prevented the greater portion of Lung Chu’s army from getting across. He then turned upon the force which had been cut off, and annihilated it, Lung Chu himself being amongst the slain. The rest of the army, on the further bank, also scattered and fled in all directions.] 5. If you are anxious to fight, you should not go to meet the invader near a river which he has to cross. [For fear of preventing his crossing.] 6. Moor your craft higher up than the enemy, and facing the sun. [See _supra_, § 2. The repetition of these words in connection with water is very awkward. Chang Yu has the note: "Said either of troops marshalled on the river-bank, or of boats anchored in the stream itself; in either case it is essential to be higher than the enemy and facing the sun." The other commentators are not at all explicit.] Do not move up-stream to meet the enemy. [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. 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.] So much for river warfare. 7. In crossing salt-marshes, your sole concern should be to get over them quickly, without any delay.