Conformity within a group entails members changing their attitudes and beliefs because of real or imagined social pressure in order to match those of others within the group. (Passer & Smith, 2003)
In general, the more members in the group, the more psychologically bonded the group, and the more the group is in close proximity, the more powerful the conformity can be. During wartime, many soldiers are gathered together, they lived in the same place and trained together within a long period of time, so they must in close proximity and they easy conform to others due to their close relationship.
So, although killing another human being is an extraordinarily difficult thing to do, especially killing those innocent citizens, during wartime, soldier feels he is isolated if he doesn't kill, and if he can get others to share in the killing process, then killing can be even easier. And soldiers clearly know that if anyone of them fails to conform to the group’s needs, he would lose credibility with the rest of the group. To fight a lone battle is very dangerous and difficult to win, so they must unite together.
In addition, we also succumb to normative social influence, conforming to obtain the rewards that come from being accepted by other people while at the same time avoiding their rejection (Deutsch & Gerard, 1955), during wartime a nation permit its soldiers to use force in order to protect the community. This permission includes lethal force because this kind of killing is for the protection and continued survival of an entire community or society. So long as a