This scenario is fair for a Rationalist. Essentially, Rationalists pursue justice through the mind. Niebuhr discusses a second group called Moralists. A Moralist believes that either through religion or through the basic universal concepts of goodness, which include a set of ethics, that if everyone lives through these, we can achieve justice. Justice is achievable through the heart. Niebuhr disagrees with the Moralists' belief because he believes it is impossible to expect everyone to act morally toward one another. He says that humans are not capable of this.
Niebuhr is an ethical realist or a Christian realist. He believes power is important and necessary to achieve greatness. The ethical realism or Christian realism is apparent in his belief that humans are not all bad. States are most important for acting. .
Niebuhr states that over time, modern society grew and became a lot more hierarchical, which created less equality. In response to people who say increasing technology fixes a lot of problems and can lead to utopia, Niebuhr states that the problems this technology has created are not being out paced by the gains. .
Niebuhr is no longer a pacifist because he states that all social order requires coercive force of some kind. Even the best forms of government, American and European democracy, involve some sort of coercive force. This force is necessary because humans have different identities, whether it is religious affiliations or wealth. It is not possible to give every person what he or she wants. He believes democracies are fairer in sorting out resources to accommodate for people's wants. Ultimately, it does not please everyone, so there need to be a threat of force.
There is a fundamental difference between individual morality and group morality. It is harder to treat a group morally, and also, vices inherent to individuals are extended to bigger groups. States are not people and do not have a sole consciousness.