Does Afghanistan even get covered by that stuff since no formal declaration of war has been made according to Hague Convention III?
IANAL but that has some problems. First of all the convention mandates 'declaration of war' in a sense that you have to tell someone you're attacking. If it was skipped then it was against the part of the convention mandating such notification.
Secondly the other parts of the convention do not talk about war as a set of specific circumstances (e.g requiring declaration of war) but just hostilities regardless on how they originated.
So by skipping the declaration of war one simply breaks one additional part of the convention, the one mandating the declaration.
I understand that changed when the UN was formed. Its member states aren't permitted to start wars, period.
The US follows the Hague Convention and other 'rules of war' regardless of if the other side does or not.
Does it matter whether both parties are signatories?
http://en.wikipedia.org/wiki/Expanding_bullet#Law
and note that domestically governments do not have to obey even the most basic "rules of war" when they set police against their own people