What were the war laws in ancient Greece?

What laws there were were set by the Amphytionic Council - consisting of delegates from the early important Greek states. The main rules: Not to destroy a city of the council members, Not to cut off the water supply of a city. Otherwise Greek warfare used every means, strategem and trick they could devise, and they were very inventive at it. The prime principle was to wage war to the ultimate, and the defeated (individual or state) were entirely at the disposal or mercy (or otherwise) of the victor (individual or state).