Chapter 69 of the Tao Teh Ching talks about the consequences of underestimating your enemy.

The strategists have a saying:
I dare not be a host, but rather a guest;
I dare not advance an inch, but rather retreat a foot.

This is called marching without moving,
Rolling up one’s sleeves without baring one’s arms,
Capturing the enemy without confronting him,
Holding a weapon that is invisible.

There is no greater calamity than to underestimate the strength of your enemy.
For to underestimate the strength of your enemy is to lose your treasure.

Therefore, when opposing troops meet in battle, victory belongs to the grieving side.