Sun Tzu's The art of war

Advice that I found interesting included:

All warfare is based on deception, hence, when able to attack, we must seem unable; when using our forces, we must seem inactive; when we are near we must make the enemy believe we are far away; when far away, we must make him believe we are near.

There is no instance of a country having benefited from prolonged warfare.

The captured soldiers should be kindly treated. This is called using the conquered foe to augment one’s own strength.

In the practical art of war, the best thing of all is to take the enemy’s country whole and intact; to shatter and destroy it is not so good.

It is the rule of war, if our forces are ten to the enemy’s one, to surround him; if five to one, to attack him; if twice as numerous to divide our army into two. If equally matched, we can offer battle; if slightly inferior in numbers, we can avoid the enemy; if quite unequal in every way, we can flee from him.

You can be sure of succeeding in your attacks if you only attack places which are undefended.

Now a soldier’s spirit is keenest in the morning; by noonday it has begun to flag; and in the evening, his mind is bent only on returning to camp.

When there is dust rising in a high column, it is the sign of chariots advancing; when the dust is low, but spread over a wide area, it betokens the approach of infantry. When it branches out in different directions, it shows that parties have been sent to collect firewood. A few clouds of dust moving to and fro signify that the army is encamping.

If those [of the enemy] who are sent to draw water begin by drinking themselves, the army is suffering from thirst.

If birds gather on any spot, it is unoccupied.

If fighting is sure to result in victory, then you must fight, even though the ruler forbid it; if fighting will not result in victory, then you must not fight even at the ruler’s bidding.

No ruler should put troops into the field merely to gratify his own spleen; no general should fight a battle simply out of pique.

Hostile armies may face each other for years, striving for the victory which is decided in a single day. This being so, to remain in ignorance of the enemy’s condition simply because one grudges the outlay of a hundred ounces of silver in honours and emoluments [i.e. for spies], is the height of inhumanity.