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

I think some of the answers here are wildly overblown. T.E.D. has the right basic idea, although ironically he only got one vote.

If you are "anxious to fight" being on the opposite side of a river is the wrong place to be, because the enemy may take a long time before he crosses it. Also, if the enemy senses that you are eager to fight him, he may delay crossing even longer on that account.

It's that simple. No need for 12 paragraphs of text.

The passage means that if you want a fight, you should not meet the enemy on his side of a river which he needs to cross. The reason is fairly obvious.

You should wait for him to cross first, and fight on your side of the river with his back to the water. If you win, you will destroy him.

Better yet, you should wait until he has crossed the river with half his force (so that he has passed "the point of no return") and then divide and conquer.

CGCampbell lays out the tactical considerations so I will add in some strategic context.

The goals of warfare in Sun Tzu's day differed massively from those in our era. Wars in those days had more in common with street gangs squabbling over turf than what we consider valid war goals. All political organizations rested on family structure and individuals who combined military and political authority did so largely based on their position in the family structure.

The goal of warfare in the vast majority of cases was kill, capture or otherwise neutralize the (usually) patriarch of the opposing side and preferably his heirs and as any other members of his extended family you could.

The armies of the day, functioned more like vast vans of bodyguards all structured ultimately to protect the lives, and if not lives, then a surviving family structure of each side. This held true, even for leaders who led from the front.

History is replete with examples of armies going from near victory to total defeat when one guy, usually the patriarch, got taken out. This caused armies to implode because not only did they lose their immediate military direction, but also the entire political and social order the army sprang out of.

Killing the leadership was likewise important because the majority of soldiers were poorly trained, equipped, and easily replaceable, so wiping out "an entire army" but not the patriarch, his family and their clients, didn't actually accomplish much militarily (and hard to do in any case when an army could just scatter.) The enemy would just round up another mob of peasants and mercenaries working on promises the next year or so and strategically, nothing changed.

Another big problem faced by the armies of the past, that we have lost an intuitive feel for, is that armies were utterly temporary assemblages that began to disintegrate almost as soon as they formed.

Most civilizations could only fight for a few months out of the year when manpower losses would not destroy agricultural output. With rare exceptions, every army in human history up until World War 2 lost most soldiers to disease than enemy action (hurrah to DDT, estimated 20 million lives saved in WW2 alone!)

Strategy in the subject era boiled down to trying to force the enemy's army to offer battle as quickly as possible, while you had some but not too much advantage, so you could get a shot at the boss. That wasn't easy because the armies were not that large relative to the terrain they operated in and one army could avoid another if it so chose.

It seems almost comical today, but heralds in the past did ride between opposing armies in order to try to negotiate a field of battle; like two groups of kids trying to decide what lot to play baseball on.

This is why sieges played such a central role in the warfare of the times. An aggressor could lay siege to a city or other specific locale that the target had to have and that could, in theory, force him to give battle to relieve the city.

Unable to lay enough sieges, Henry the Fifth and his heirs, adopted a massive series of raids or Chevauchée that destroyed so much of the French kings agricultural lands, they had no choice but to come out and give battle. Basically, "come and fight where we can get a chance to kill you or we burn down the entire neighborhood!"

In Chapter 9 (The Army on the March), Verse 5, Sun Tzu is advising the aggressor, not a defender, in this case and dealing with the tricky problem of inducing an enemy army to cross a river to give battle, a crossing which the enemy knows will place them at risk while transiting.

Basically, his advice boils down to, "don't be tempted to defeat the enemy's bodyguard in detail as they cross because they'll just stop crossing and go away. Instead, step well back, then let them get all across, and then you'll get your chance to kill the big boss."

There were exceptions to the use of this tactic. In some cases, the destruction of the army was the point. The Battle of Bannockburn comes to mind. In it, the Scots wanted to destroy a relieving army so they could gain the castle Sterling. The odds of defeating the entire English army were slim and killing the King far more so. They just needed to kill enough of the relief force to make relief impossible and they would win. Fortunately, defeating a part of the English army in detail caused a rout, turning a wise, conservative, tactical action into a stunning strategic victory.

Very telling, if I recall correctly, Sun Tzu did not advocate these tactics with nomads but instead used bodies of water to hold them off until they got tired and rode back to the steppes. Nomads were decentralized, even under the great Khans, and killing one guy seldom caused wholesale collapse.

Sad to say, but a lot of what passed for large-scale chivalry back in the day, in most civilized cultures, often boiled down to politely cajoling one's enemy to stick his head in the noose before the clock ran out. More of a "hey, come out here where I can kill you before everyone sobers up and wanders away," than something out of Camelot.

I don't have any quotes handy and can't comment yet. However I remember another concept Sun Tzu suggests:

• When an army has no retreat, the individual fighters will defend themselves more viciously.

It may seem counter-intuitive but soldiers hold back even when defending themselves. They fear being injured and keep looking for ways to retreat. The idea is to use this by intentionally getting yourself into a position literally with your back to a wall.

If you attack while the enemies have their back to a river, the enemy army has this particular advantage. You shouldn't grant it to them. Instead let them have a way to retreat. It may dismantle their morale and work for you.

I'm not sure about encircling the enemy. Completely encircling would stretch out your lines and again give the morale boost of "no way out" to the enemy. But I guess that's a different question to ask.

My interpretation is that the basic idea here is that the force that crosses the river is at a huge disadvantage. Based on that, there are two obvious problems with putting yourself on the opposite bank of a river when you are "anxious to fight". The first is that this may well delay things greatly, as a sensible general will not be quick to make a contested crossing. The second would be the danger of the other force somehow inducing you to be the one to make the crossing.

That being said, I should point out something about Sun-Tzu interpretations. I bought an edition with footnotes from famous ancient Chinese generals. One of the things I noticed fairly quickly was that quite often their interpretation of a passage was completely different than mine, and than each other's as well.

Sometimes I'd even see a note arguing that a passage said the exact opposite of what it clearly seemed to be saying. The zen-ish bits about the best general being one who doesn't have to fight seemed to particularly cause the historical generals fits (some of them really liked fighting I guess). The conclusion I came away with was that the notes were actually way more enlightening about the minds of those other generals than they were about Sun Tzu.

The point here is that any interpretation you get is just that: someone's interpretation. Quite often there's no one right answer.

Verse 3. After crossing a river, you should get far away from it.

If the river is a barrier, you can be hemmed in against it. If your enemy is the one hemmed in, they also have a defense on at least one side, preventing you from surrounding them.

Verse 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.

If you meet the enemy as they begin to cross, they may halt their crossing, preventing the attack. They may fire at you from the far shore.
If you let only as much of the enemy force as you can handily defeat, cross the river, you can easily defeat them while the reinforcements attempt to cross to their comrades.
This is a form of "divide and conquer."

Verse 5. If you are anxious to fight, you should not go to meet the invader near a river which he has to cross.

If your enemy has notice of your approach and are on your side of the river, they can cross the river first, and then use verse 4 against you.
If your enemy is on the far side of the river, and they have notice of your approach, they can deny your attack. If they don't have time to cross, they can use the river as a defense against encirclement (verse 3).

These three verses are generally thought to be supportive of one another.