This observation means that nations, states, collectives, or other groups do not act, but rather these abstractions can only preform actions by the actions of individual humans. It is a metaphor to say that the American military invaded Afghanistan. The metaphor is useful as long as we understand that it was really an invasion of many individual humans each preforming individual tasks that went to make up the overall invasion. Each human is responsible for his own actions.
The ancient view of the state was that special rules of morality were used to judge the State, if it could even be morally judged at all. The State was above the moral judgments that we apply to individual men. This would allow an individual to kill, rape, bomb, torture, pillage, or humiliate others in the name of his Nation State with moral impunity. The individual could remain “moral” as long he committed these crimes in service of the state.
The Classic Liberal tradition abolished this foul idea and replaced it. The liberal idea was that the nation state was to be judged in the same way as the individual human. This new idea of moral judgment changed the face of politics. It also meant that individual humans were to be judged based on their individual actions, even if in service of the collective. No longer would “I was following orders” be an acceptable excuse.