War of Ferrara The War of Ferrara ( also known as the Salt War , Italian : " Guerra del Sale " ) was fought in 1482 – 1484 between Ercole I d ' Este , duke of Ferrara , and the Papal forces mustered by Ercole ' s personal nemesis , Pope Sixtus IV and his Venetian allies . Hostilities ended with the Treaty of Bagnolo , signed on 7 August 1484 . Diplomatic background . The failure of the Pazzi Conspiracy against Florence in 1480 and unexpected peace as a result of Lorenzo de ' Medici ' s daring personal diplomacy with Ferdinand I of Naples , the Pope ' s erstwhile champion , was a source of discontent among the Venetians and Pope Sixtus IV alike . Venice had ended its long conflict with the Ottoman Turks in 1479 , with the Treaty of Constantinople , and was freed to turn its complete attention to its role in its " terra firma " ( main land ) and generally to the peninsula of Italy . In addition to the usual minor friction over strongholds along the borders , there was a contest over the commerce in salt , which was reserved to Venice by a commercial pact . Nevertheless , Ferrara , which was ruled by Ercole I d ' Este , had begun to take control over the saltworks at Comacchio . This appeared to be a threat to mainland interests of the Serenissima . Venice was supported by Girolamo Riario , lord of Imola and Forlì , – the nephew of Pope Sixtus – who had taken possession of the strategic stronghold of Forlì in September 1480 , with quick papal confirmation , and who now looked towards Ferrara in extending Della Rovere territory . The immediate " casus belli " at the beginning of 1482 , as usual was a minor infraction of prerogatives : Venice maintained a representative in Ferrara with the high title of " visdominio " , under whose care was the Venetian community in Este lands . In 1481 , overreaching his mandate with the arrest of a priest for debt , the " visdominio " was excommunicated by the vicar of the bishop of Ferrara , and forced out of the city . On this excuse , war was declared . Allied with Venice , besides the papal troops and Riario , were contingents supplied by the Republic of Genoa and William VIII , Marquis of Montferrat . Taking Ferrara ' s side , loosely under the command of Federico da Montefeltro , duke of Urbino , were troops of Ercole ' s father - in - law Ferdinand of Naples , under his son Alfonso of Calabria , who invaded the Papal States from the south , as well as troops sent by Ludovico il Moro of Milan and by the lords of two cities threatened by the mainland power of Venice , Federico I Gonzaga of Mantua and Giovanni II Bentivoglio of Bologna . The course of the war . Venetian troops led by the " condottiero " Roberto Sanseverino attacked Ferrarese territory from the north , brutally sacking Adria , quickly overrunning Comacchio , attacking Argenta at the edge of the saltmarshes and besieging Ficarolo in May ( capitulated 29 June ) and Rovigo ( capitulated 17 August ) . Venetian forces crossed the Po River and in November 1482 stood before the walls of Ferrara and laid strict siege to the city . Sixtus appears to have had a change of heart at the season ' s advances , which now threatened to put Venice in an uncomfortably strong position in mainland northern Italy . In the Papal States , the Colonna family took advantage of disorder , and fought pitched battles against their Della Rovere enemies . The main encounter , however , was the pitched Battle of Campomorto near Velletri , 21 August 1482 , in which the Neapolitan troops were soundly defeated by Roberto Malatesta and the duke of Calabria was barely rescued by a contingent of his Turkish soldiers . Some Orsini castles also fell into papal hands , but where battle failed malaria succeeded : Roberto Malatesta ' s death in Rome on 10 September largely unraveled Papal successes in the Lazio . Sixtus made a separate peace with Naples in a truce of 28 November and a peace treaty was signed on 12 December . His entreaties with Venice to cease hostilities were vigorously rebuffed , with threats of excommunication countered by the withdrawal of the Venetian ambassador , followed by Sixtus ' interdict of Venice in May 1483 . Now Sixtus granted free passage to Alfonso and his troops to go to defend Ferrara against the pope ' s recent allies , aided by papal troops under Virginio Orsini . A contingent of Florentine troops arrived also , and the fortunes of Este began to look much better . In a diversionary maneuver , Venice sent Roberto Sanseverino to attack the Duchy of Milan under the pretext of supporting the rights of the Visconti heir , but the diversion was further diverted when Robert had to counter Alfonso , who was sacking Milanese territories . The war began to lose momentum . The Treaty of Bagnolo . The war came to a conclusion with the Treaty of Bagnolo , signed on 7 August 1484 . Ercole ceded the territory of Rovigo in the Polesine , lost at an early stage of the fighting , and the Venetian forces that were occupying Ferrara - owned territory withdrew . Ercole had successfully avoided the absorption of Ferrara , seat of the Este , into the Papal States . Sixtus was rendered more eager to sue for peace by the series of victories by Venetian forces , who seized the opportunity to forward their territorial ambitions and had been hasty to declare war on Ferrara on a minor pretext . Florence , Naples , Mantua , Milan , and Bologna stood by Ferrara . While the papal forces were holding in check the Neapolitans who sought to move north to aid Ferrara , and with the Roman Campagna being harassed by the Colonna , and Milan engaged in combat with Genoa , the Venetians had besieged Ferrara into starvation . With the Venetians ready to take over Ferrara , the Pope , fearing his erstwhile allies , suddenly changed sides : he made a treaty with Naples , and permitted the Neapolitan army to pass through his territories , giving them the chance to convey supplies to Ferrara and neutralize the siege . At the same time the Pope excommunicated the Venetians , and now urged all Italy to make war upon them . The Peace of Bagnolo checked Venetian expansion in the " terra firma " , ceding to it the town of Rovigo and a broad swath of the fertile delta of the Po . This acquisition agreed upon at Bagnolo marked the high - point of Venetian territory ; never again would Venice control so large a territory nor have so much influence as it did in the last half of the 15th century . Nevertheless , Sixtus was not pleased with the terms reached without consulting him : The war was the subject of an anonymous poem entitled " La guerra di Ferrara "