TBH How is this any different from an defending army salting the earth back in ancient times to prevent pillaging from an invading force. Or an invading force poisoning a water supply. Reality of it is, the term "rules of war" back in ancient times, including the middle ages, was extremely vague. It wasn't until the birth of full on chemical warfare during World War 1 that every major country decided that it was a really bad idea to do it considering the potential fallout involved.