This dissertation deepens the understanding of John Paul II's social thought within the perspective of Catholic Social Teaching. The dissertation explores John Paul II's distinctive contribution to the teaching of the Roman Catholic Church on the ethics of war and peace with particular attention to his interpretation and application of the norms of the just war tradition. The introduction presents this issue in general and the dissertation's methodology, which includes the just war theory (jus bellum justum) as the systematic framework for the comparative method and analysis. Catholic Social Teaching on war and peace is presented in its chronological development and systematized into four categories of jus bellum justum: jus ad bellum, jus in bello, jus ad bellum/in bello, and jus post bellum. Such material was analyzed in the historical context - hermeneutically interpreted -- from the perspective of the contribution of each pope from Leo XIII to John Paul II, and of the Second Vatican Council. Chapter 1 presents the historical background of the Church's reflection on war and peace and important elements taught by Leonine popes from Leo XIII to Pius XII (pre-Vatican II era). Chapter 2 presents the significant development of this teaching made in Vatican II and the post-Vatican II era by popes from John XXIII to John Paul I. Chapter 3 presents John Paul II's teaching on war and peace and reveals the significant distinctive elements of his teaching in comparison to his predecessors and Vatican II. Chapter 4 analyzes these distinctive elements as a leading contribution of John Paul II: the novelty of humanitarian intervention as a new criterion of just cause, the role of forgiveness and social reconcilliation in building peace in the new millennium, and defining solidarity as a social virtue overcoming structures of sin, which is the basis for realizing both humanitarian intervention and forgiveness on the way to reconciliation. The growing awareness of interdependence among people and a concern for solidarity are new signs of peace. Humanitarian intervention and forgiveness are the first two new examples of the social-political virtue of solidarity given by John Paul II.