This thesis considers changes in the diplomacy of the Holy See with respect to the Middle East in the period between 1990 and 2003. Policies pursued by these changes were decisions of Pope John Paul II and involved (1) establishing full diplomatic relations between the Holy See and the State of Israel; (2) convening the Special Assembly of the Synod of Bishops for Lebanon, ending in the papal visit to Lebanon in May 1997; and (3) opposing the 1991 and 2003 U.S. led wars against Iraq. Asking whether ideas or interests explain these policies, the thesis argues that new circumstances occasioned a rethinking of the Holy See's interests in light of the development of modern Catholic social teaching. In other words, ideas constituted interests. Principled belief in the dignity of the human person and the principle of solidarity shaped the Holy See's perception of its interests and the substance of the Pope's diplomacy. In making the argument, the thesis considers the Holy See's role in international relations; the development of modern Catholic social teaching since Pope Leo XIII's 1891 encyclical, Rerum Novarum; and case studies of John Paul II's diplomacy with respect to Israel, Lebanon, and the wars with Iraq.