Yes, there are laws of war!

Purposes of the laws

Some of the central principles underlying laws of war are:

• Wars should be limited to achieving the political goals that started the war (e.g., territorial control) and should not include unnecessary destruction;
• Wars should be brought to an end as quickly as possible;
• People and property that do not contribute to the war effort should be protected against unnecessary destruction and hardship;

To this end, laws of war are intended to mitigate the evils of war by:

• Protecting both combatants and noncombatants from unnecessary suffering;
• Safeguarding certain fundamental human rights of persons who fall into the hands of the enemy, particularly prisoners of war, the wounded and sick, and civilians;
• Facilitating the restoration of peace.

http://en.wikipedia.org/wiki/Laws_of_war