CASTRO, WAR OF. — The pontiff Paolo III had granted his relatives, the Farnese, in addition to possession of Parma, that of a small duchy in the Viterbese, called C. after its principal settlement, enclosed within the territories of the Papal States. At the time of Urbano VIII, the Barberini, avaricious nephews of the reigning Pope, viewed with disfavour the presence of outsiders at the gates of Rome; for his part, Odoardo Farnese, duke of Parma, had various reasons for resentment against the Pontiff and the new lords. In such circumstances it was easy to find a casus belli, and the war soon became a test of the political prestige of the Papal State, involving all the other Italian States, which were glad to strike a blow at ecclesiastical power.
In January 1640 Farnese placed C. on a war footing; Urbano demanded payment of the interest on the loans that Roman bankers had granted him, securing themselves with the revenues of the duchy. When he refused, the Pope ordered that traffic between Rome and Tuscany be diverted from the road passing through C. and prohibited grain exports; the rebel did not yield, and open war broke out (late 1641). Under the command of the marquis Luigi Mattei, the papal troops moved from Viterbo against C. and occupied it almost without striking a blow; this was followed by Farnese’s excommunication and the confiscation of his allodial possessions, including his palace in Rome. But, disregarding counsels of moderation, Urbano VIII then wished to continue the enterprise by turning against Parma and Piacenza, feudal dependencies of the Holy See; alarmed, the Italian powers joined in a league and provided financial assistance. Odoardo was thus able to march at the head of an army «against the Barberini», as he said, not against the Pope, and to occupy Bologna, Imola, and Forlì. In Rome a new sack was already feared, like that of Borbone, but Farnese was unable to profit from the favourable circumstances. At this point the question became entwined with the various European controversies and the perpetual France–Spain duel, in that the French ambassador in Rome sought to make peace solely in order to direct the Italian forces against the duchy of Milan, which was held by the Spaniards; in the spring of 1643 a league was concluded among Venice, Tuscany, the Este, and Farnese to «restore Italian peace», as was often repeated at the time, but in substance to derive the greatest possible advantage for each party, amid profound mutual distrust. Meanwhile the war continued, almost without human casualties but with great financial damage, until Mazarino commissioned Cardinal Alessandro Bichi to conduct peace negotiations separately with the various parties involved, in order to prevent Spain and the Empire from also being present at a general congress; the defeat of the papal troops at Lagoscuro at the hands of the Venetians (spring 1644) hastened the conclusion. Territorially, the status quo ante was restored, but the Pope’s moral position, having been held in check by a rebellious subordinate, emerged irreparably shaken; while Urbano, after publishing the peace on 1 May 1644, died on 29 July of the same year.