Doing Battle - Sun TZU's The Art of War

 
   When you do battle, even if you are winning, if you continue for a long time it will dull your forces and blunt your edge; if you besiege a citadel, your strength will be exhausted.  If you keep your armies out in the field for a long time, your supplies will be insufficient.

     Arms are tools of ill omen--to employ them for an  extended period of time will bring about calamity.  As it is said, "Those who like to fight and so exhaust  their military inevitably perish."

     When your forces are dulled, your edge is blunted, your strength is exhausted, and your supplies are gone, then others will take advantage of your debility and rise up.  Then even if you have wise advisers you cannot make things turn out well in the end.

     Therefore I have heard of military operations that were clumsy but swift, but I have never seen one that was skillfull and lasted a long time.  It is never beneficial to a nation to have a military operation continue for a long time.

  ...

  Therefore, those who are not thouroughly aware of the disadvantages in the use of arms cannot be thouroughly aware of the advantages in the use of arms.

Sun TZU