Verse 31 of Tao Te Ching by Lao Tzu

It is because arms are instruments of ill omen and there are Things that detest them that one who has the way does not abide by their use.
The gentleman gives precedence to the left when at home, but to the right when he goes to war. Arms are instruments of ill omen, not the instruments of the gentleman. When one is compelled to use them, it is best to do so without relish. There is no glory in victory, and to glorify it despite this is to exult in the killing of men. One who exults in the killing of men will never have his way in the empire.
On occasions of rejoicing precedence is given to the left; on occasions of mourning precedence is given to the right. A lieutenant's place is on the left; the general's place is on the right. This means that it is mourning rites that are observed.
When great numbers of people are killed, one should weep over them with sorrow. When victorious in war, one should observe the rites of mourning.