69.1 There are sayings on the use of arms:
"Let us not be aggressors, but defend."
"Let us not advance an inch, but retreat a foot."
69.2 This is called carrying out no action,
shaking no arm,
facing no enemy,
wielding no weapon.
69.3 No calamity is greater
than underestimating opponents.
If you underestimate opponents,
you're close to losing your treasure.
69.4 So when opposing armies clash,
the compassionate are the ones who win.
Tao Teh Ching - Cleary Translation
The "treasure" referred to in line 69.3 does not refer to material resources or human lives, but to the virtues mentioned in Chapter 67 -- mercy, restraint, and humility -- to which we could add truth, the first casualty of war.
Post-traumatic stress syndrome, a greatly under reported and under treated effect of war, would be called loss of soul by the Navaho who would have blessing ritual for the returning veteran to reunite him with his soul and his community.
Even when war is conducted with due regard to spiritual concerns, something dies inside a person who has killed an enemy combatant, or worse, a noncombatant. The people who recognize that something bad has happened to them are healthier than those who don't.
So the sage recommends restraint even in the heat of battle -- especially in the heat of battle.
There are many ways of underestimating opponents, their firepower, their skill, their numbers, their strategic and tactical genius, their discipline. I don;t think the sage is thinking about these things. What i am most likely to underestimate in my enemy is his humanity, his goodness, his likeness to myself.
The compassionate "win" because they do not lose their "treasure."
Prayer: Holy Loving: Help me discover peace within the war which is 21st century Earth. Sometimes i can do nothing except try to pray that my life be ordered in a peaceable way. May that be my prayer. Amen.