Conflicts involving religion have returned to the forefront of international relations. And yet political scientists and policymakers have continued to assume that religion has long been privatized in the West. This secularist assumption ignores the contestation surrounding the category of the secular in international politics. The Politics of Secularism in International Relations shows why this thinking is flawed, and provides a powerful alternative. Elizabeth Shakman Hurd argues that secularist divisions between religion and politics are not fixed, as commonly assumed, but socially and historically constructed. Examining the philosophical and historical legacy of the secularist traditions that shape European and American approaches to global politics, she shows why this matters for contemporary international relations, and in particular for two critical relationships: the United States and Iran, and the European Union and Turkey. The Politics of Secularism in International Relations develops a new approach to religion and international relations that challenges realist, liberal, and constructivist assumptions that religion has been excluded from politics in the West. The first book to consider secularism as a form of political authority in its own right, it describes two forms of secularism and their far-reaching global consequences.