For Niebuhr, the ideal of a just society will never be realized because it is a vision rooted in the conscience of the individual -- who has resources for living in groups that are not available to the groups themselves. These two resources are reason and conscience. Groups lack a collective conscience and often use reason for their own purposes. (Niebuhr does say that individuals use reason for their own self-interest, but they are, at times, able to transcend themselves and use reason altruistically.) Because groups lack the moral resources that individuals possess, we must be content with a modest goal of approximating justice through political means. This requires some form of coercion.
The most prominent group that uses coercion to secure justice is the nation-state. Niebuhr argues that the nation makes two contradictory and incompatible claims on the individual: (1) that the nation is a special, unique community -- which appeals to the emotion of the individual -- and (2) that the nation embodies universal values and ideals -- which appeals to the mind of the individual. In each case, the individual loses their prophetic sensibilities toward the nation. Further, Niebuhr argues that securing justice through coercion might require some form of violence. He is clear to point out that non-violence is a preferred posture, but that violence cannot be ruled out a priori as a means to securing a good end. For Niebuhr, violence is justified if it proceeds from perfect moral goodwill -- once an end is justified, the means to achieve that end become pragmatic. The key criterion for Niebuhr is the potential political possibility of establishing justice through violence.
Niebuhr spends the middle three chapters examining the Marxist political alternative given this reality of the distinction between individual morality and group morality. He sees some potential in a progressive Marxism, but ultimately finds it much like religion -- focusing on the ideal and losing the ground gained in reality and approximations. Establishing justice within and among groups requires a realistic assessment of the resources available to the individuals of the group and how those resources play to the self-interest of the group to preserve its power and influence on society.