The first of four campaigns against the territorial ambitions of Filippo Maria Visconti, duke of Milan, was connected to the death of the lord of Forlì, Giorgio Ordelaffi.

The young Luigi degli Alidosi was sent captive to Milan and a few days later the lord of Faenza, Guidantonio Manfredi, joined the Visconti party.

By the agreement the war was to be pursued at the common expense of both: the conquests in Lombardy to be assigned to the Venetians; those in Romagna and Tuscany to the Florentines.

Visconti regained the lands occupied by Florence in Liguria, but had to renounce the area of Vercelli, conquered by Amadeus VIII of Savoy, and Brescia, which went to Venice, and to promise to stop encroaching himself in Romagna and Tuscany.

The Venetian commander pushed him back and conquered Casalmaggiore on 12 July, while Orlando Pallavicino, lord of several castles near Parma, rebelled against the Visconti as Amadeus VIII and John Jacob of Montferrat invaded Lombardy from East.

The troops sent to reduce that city, under Niccolò Fortebraccio, were later sent to invade the Lucca, whose lord, Paolo Guinigi, had previously sided with the Visconti.

The subsequent defeat on the Serchio banks of their commander Guidantonio da Montefeltro (2 December 1430), encouraged the Florentines to engage the aid of Venice once more and re-erect their lapsed League, with the favour of the new Pope, Eugene IV, a Venetian.

The League's army was first beaten at Soncino (17 May 1431), while Luigi Colonna defeated the Venetians at Cremona, Cristoforo Lavello pushed back the Montferrat troops, and Piccinino established strong positions in Tuscany.

Venice won a naval victory over Genoa at San Fruttuoso on 27 August 1431, but on land Carmagnola, the commander of Venetian forces, moved cautiously, avoiding a pitched battle and raising the suspicion he could have been bought by Visconti, while the latter was also joined by Sigismund who had entered Italy to receive the imperial crown.

In the end Carmagnola was suspended; recalled by the Council of Ten, he was arrested in March 1432, tried for treason and beheaded outside the Doge's Palace.

The Florentine war with Lucca and her allies likewise resulted in a return to the previous status quo, but the major League leader's lack of successes had lost much charisma: the Venetian doge Francesco Foscari was on the verge of resigning, while Cosimo de' Medici was imprisoned and confined in Padua.

In the so-called "fourth war" broader questions were personalized in the combats among antagonistic condottieri: Gattamelata, and later Francesco Sforza fought nominally for Venice, while the Visconti side was led by Niccolò Piccinino, who had promised to Eugene IV to reconquer the Marche for him.

In response to this Venice signed an alliance with Florence and Francesco Sforza, including some notable captains of the time such as Astorre II Manfredi and Niccolò III of Ferrara, who was also restored the Polesine in exchange for his support.

No large territorial changes were made in the ensuing Peace of Cremona of 20 November 1441: Venice kept Ravenna, Florence the Casentino.

Piccinino was awarded the lands of Orlando Pallavicino in the Parmense, while Filippo Maria Visconti recognized the independence of Genoa and again promised to stop interfering with the situation in Tuscany and Romagna.

[e] Two coalitions now formed: Sforza's Milan allied with Medici's Florence on the one hand, faced Venice and the Aragonese Kingdom of Naples on the other.

In April 1452, the Republic of Venice declared war on the Duchy of Milan, who responded by crossing the river Adda and invading the counties of Bergamo and Brescia.

In April they reached the Treaty of Lodi, a compromise peace that formed the basis for a general accord among the four contenders, Venice, Milan, Florence and Naples, under the blessings of Pope Nicholas V, representing the fifth power in Italy.