This paper opens a window on the relations between Rome and Genoa during the Castro crisis in the 1640s. The war between Rome and an alliance of Italian States underscored Genoa's complex positioning in the Italian geopolitical context. On the one hand were the private interests of Genoese groups tied to the Barberini, pushing for an alliance with Rome; on the other was observance of a traditional neutralism of the Republic, which had no significant military forces and no direct interest in war. Between official neutrality and the active involvement of private groups, Genoa was a political and diplomatic battlefield animated by secret movements in support of Rome and the Barberini (even after the end of the pontificate of Urban VIII), and by ambitious propagandistic strategies implemented by both sides to push the Republic into war. The production of pamphlets and the activism of a prince directly involved in the conflict, Francesco d'Este, encouraged communication with regard to political issues of importance to the Republic (the recognition of the royal honours), which was subjected to the Spanish protectorate and had to use private and informal instruments of negotiation with extreme caution.