War crimes most certainly exist. The US does have certain obligations. For example, IMO, it would be a war crime to kill captured AQ agents without a trial or to torture them. There are international treaties and US domestic laws making such kinds of conduct a crime. Both nations and individuals, however, have a right to defend themselves from those wishing to attack them. That's true whether the assailants are war enemies or criminals. There's no reasonably debatable argument going on in any international legal circles about the nature of AQ. It's an illegal terrorist organisation in every country of which I'm aware, and even the UN has consistently labelled them such and has routinely authorised decrees, sanctions, use of force authorisations, etc, aimed at expunging AQ. So, IMO, 'assassination' is not really a good term for killing AQ members. They are members of a known threat and illegal terrorist organisation and each member of the conspiracy is therefore a known threat as a member thereof. They make the illegal choice to join it and pursue the illegal ends of attempting to kill and visit destruction upon the US and many other nations and people, and therefore it's the right of AQ's targets to defend themselves against them, including killing them before they kill their targets. If AQ members wish not to be killed, they can quit being members or surrender to authorities. But so long as they remain in the public and seek to avoid capture in order to attack the US and anyone else in their plans, they make themselves legitimate targets for being killed in self-defence.