9.4

Chapter: The Army on the March 9.4
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Translation:

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.

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Commentary

If you try to meet the enemy as they start crossing the river, they may retreat or fire at you from the other bank.

If you let as much of the opposing force cross that you can easily defeat, then you can complete this task whilst the remaining force struggles to cross the river.

By dividing their army in this way, you can defeat a superior force. This is just one variant on the ‘divide and conquer’ principle.