Attacking the World Trade Centre involving the deaths of people from all countries was surely very much an attack on the world as a whole, specifically an attack on America.

As a criminal act carried out on United States teritory there is obviously good reason for the relevant trials to be carried out under the American legal system in America.

In fact I can't see that it would count as a war crime, which is as well. War crimes tend to go unpunished, which is something I would like to see changed, which is why I would like to see an International War Crime Tribunal set up, which is something that almost all civilised governments have indicated that in principal they are agreed on.

But that's got nothing to do with Walker's involvement with the Taliban, since there doesn't seem any reason to think he was a terrorist. Being on the losing side in a war is not in itself a war crime.