During the First War of Castro he served as plenipotentiary of Pope Urban VIII; sent to negotiate a truce with the Duchy of Parma, together with his brother Virgilio. Spada successfully negotiated a truce but when the pope's military leaders became aware that the dukes were massing troops to counter their troops (in case discussions with Spada came to naught), Urban VIII declared the articles of peace null and void and claimed Spada had negotiated them without his consent. Spada was furious and later published a manifesto detailing his version of events which, according to contemporary John Bargrave, many accepted to be the truth.