Tao te Ching: 31

Fine weapons are instruments of evil.
They are hated by men.
Therefore those who possess Tao turn away from them.
The good ruler when at home honors the left (symbolic of good omens).
When at war he honors the right (symbolic of evil omens).
Weapons are instruments of evil, not the instruments of a good ruler.
When he uses them unavoidably, he regards calm constraint as the best principle.
Even when he is victorious, he does not regard it as praiseworthy,
For to praise victory is to delight in the slaughter of men.
He who delights in the slaughter of men will not succeed in the empire.
In ausipcious affairs, the left is honored.
The lieutenant-general stands on the left.
The senior general stands on the right.
That is to say that the arrangement follows that of funeral ceremonies.
For the slaughter of the multitude, let us weep with sorrow and grief.
For a victory, let us observe the occasion with funeral ceremonies

You know, it's said that the line about the arrangement of funeral ceremonies was just a side note, not the original text? You have to be wary of the text. It is thousands of years old and subject to interpretation.

Notice how Lao Tzu doesn't say to be a pacifist. He doesn't say not to use weapons or force. He says they are hated and thsoe that have Tao turn away from them, yet there are times when men must take up arms. Remember that the left is the black, the feminine, the receiving side and so the opposite is true of the right. In war, stay to the right, the masculine, the acting side but do not regard it as praiseworthy. Though you have to act in the right, remember the left if you are to stay with the Tao.