Just War, first articulated by St Thomas Aquinas, made clear you go to war when your cause is ‘just’ and you conduct war in such a manner as to minimize suffering of the innocent.
“The war must be fought proportionally.
This means do not use more force than necessary or kill more civilians than necessary.”
http://h2g2.com/dna/h2g2/alabaster/A644672
http://www.h2g2.com/approved_entry/A644672
http://www.freerepublic.com/focus/f-religion/1996394/posts