How can citizens of a political society be united? What makes them willing to sacrifice for the good of the community? How are they made to obey the laws? The ancient world approached these questions through concepts such as virtue and honor. Alexis de Tocqueville’s Democracy in America also attempted to answer these questions in relation to newly emergent democratic societies. However, he developed new concepts to formulate his answers, including enlightened self-interest, individualism, and free associations. Essential to his argument is the role of changing social conditions. For Tocqueville, the forces which have shaped the modern world, such as democracy, have made the ancient concepts irrelevant. Indeed, the changes which he had witnessed were so revolutionary that he was compelled to say, “I am tempted to burn my books so as to apply only new ideas to a social state so new” (I.2.9, 289). It thus becomes necessary to conceive of new ways of organizing cohesive political societies. This thesis builds on Tocqueville’s theories and observations to explain how changing social conditions can shape the citizen’s ability to cooperate as part of a cohesive polity and how modern societies can promote harmony among its citizens. I first explore briefly how the ancient world inspired citizens to work cohesively and how modern changes in ideas, sentiments, and mores have challenged the efficacy of premodern traditions. I then analyze how modern conditions can limit attempts at political cohesion and the challenges of promoting acts of solidarity among modern citizens. I also consider how democratic despotism offers a vague form of political cohesion that conforms to modern conditions, but in ways that undermine good governance. Finally, I argue that Tocqueville’s theory of enlightened self-interest, bolstered by a religious spirit that combats materialism, offers the most coherent account of how modern political societies can be united justly and how citizens can act harmoniously toward a common good. While enlightened self-interest and religion may be goods within themselves, this thesis suggests that these principles are also necessary for creating cohesion in the modern age.