No ruler should put troops into the field merely to gratify his
own spleen; no general should fight a battle simply out of pique.
If it is to your advantage, make a forward move; if not, stay
where you are.
Anger may in time change to gladness; vexation may be succeeded
by content.
But a kingdom that has once been destroyed can never come again
into being; nor can the dead ever be brought back to life.

Author: Sun Tzu