A country is an abstract concept. On the battlefield, it is represented by its soldiers in uniform. There is no Uncle Sam in a uniform who represents the "US" collectively.
If I take your argument to the extreme, it'll be tough to hold any army anywhere in the world to account. Because they can always attribute it to "individuals".
Exactly. Which is why he is asking for evidence that there is a currently US military policy in place that states soldiers are to intentionally target civilians in the "war on terror".
Individuals acting on their own committing crimes is much different than an official policy, ordered to be enforced by the military chain of command.
If I take your argument to the extreme, it'll be tough to hold any army anywhere in the world to account. Because they can always attribute it to "individuals".