Why did Sun Tzu believe you shouldn't wait beside a river when eager to fight?

Question

In Sun Tzu's Art of War, Chapter 9 (The Army on the March), Verse 5, Tzu says > If you are anxious to fight, you should not go to meet the invader near a river which he has to cross. My thinking is that you should meet the invader near a river and let him cross it so that he is tired and weak. I am confused as to why "you should not meet the invader near a river which he has to cross."

Answer 1

Yeah, this is one of his more subtle ones, isn't it? Most of his advice makes excellent sense, even if it sometimes needs explaining first. What he meant this time was that an enemy crossing a river might be trying to draw you close so a hidden force that crossed earlier -- say, in the night -- could ambush you while the larger force closes in from the river.