Historically, Niebuhr’s significance is how he finessed the pre-World War II pacifism of the mainline churches. He argued that modern society was not utopian, that appeals to reason or altruism wouldn’t work because power was how things got done.
His work “Moral Man and Immoral Society” argued (to simplify dramatically) that, realistically, in order to do good, the moral person might have to use means that are immoral. Or, to put it another way, in order to resist a great evil, the moral person might have to embrace means that are still evil, but less so.
His reasoning was reassuring for persons looking at the rise of Nazism in Europe and Japanese fascism in Asia, but who also felt that war was wrong. For many Christians, he resolved their moral qualms about fighting in World War II.
Christians (and Jews) will find it well worth reading.