The author analyses the legis actio per condictionem procedure, as a basis for the development of unjustified enrichment claims (condictiones) in classical Roman law. He specifically deals with the origins of this procedure, which is still an unresolved issue in Romanist studies. He thoroughly analyses the basic characteristics of this procedure: the thirty-day time period (dies tricensimus) applied in the procedure and the usage of the terms condicere and condictio, upon which the name of this legis actio is based. Finally, he defines the terminological and conceptual interconnection of the legis actio per condictionem procedure with the ancient Roman procedure for a declaration of war, or the bellum iustum procedure. Due to certain common features of these two procedures, the author suggests a similar level of their immersion in the ancient Roman religious and legal concepts. That way, he contributes to the understanding of the origins of the legis actio per condictionem procedure, which in turn helps with the analysis of the unjustified enrichment claims (condictiones) in Roman law and its reception in general.