While the evangelical forces had labored to realize their “true American Union of Church and State,” vast changes had been taking place within American society. Large contingents of hitherto unrepresented ethnic groups—frequently locating their identities through their Roman Catholic or Jewish religious practices—were arriving year after year. The frontier was relentlessly conquered, and urban areas were developing concurrently. Inventions and investments transformed the American economy and with it American society. Also a fratricidal conflict disclosed deep conflicts within the United States. Thus, although the Protestant revivalistic endeavors continued and the united front activities proceeded, the premise on which they had originated did not have continuing validity. The United States was no longer overwhelmingly Protestant. It was ceasing to be in large part rural. It would never again have the potential to be a Christian Republic—the vision that had inspired the evangelical efforts. Thus the years between the Civil War and World War I represent a period of transition in some ways analogous to the first half of the eighteenth century—at least when viewed in the perspective of Church and State. The responses of Protestantism during these years to the changes in American society were manifold, and no such nearly universal consensus as republican Protestantism began to develop. In this section our concern is primarily with the kinds of tension between the religiously plural society coming into existence and the older paradigms of Church-State relations—especially republican Protestantism but also continental Roman Catholicism—which did not readily comprehend either the situation of post-Civil War America or the legacy of the Constitutional epoch.