The Coetus Internationalis Patrum (CIP), chaired by Mgr. Lefebvre, was the main group of opponents to the general spirit of Vatican II. In this article, the author briefly exposes his research on the subject at hand; this research culminated in the writing of a doctoral thesis defended on November 3, 2011. After making a history of research on the topic at hand and having presented the sources used, the author describes the organization, activity and CIP strategy, to then evaluate its impact on the Council schemes.