Some background from wikipedia to the 1st War in Lombardy(which is probably unnecessary in translating text):
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. He had named Visconti the trustee of his nine-year-old heir, Teobaldo II. The latter's mother, Lucrezia degli Alidosi, daughter of the lord of Imola, did not agree and assumed the regency for herself. The Forlivesi rebelled and called in the city the Milanese Visconti's condottiero, Agnolo della Pergola (May 14, 1423). Florence reacted by declaring war on Visconti. Its captain Pandolfo Malatesta therefore entered Romagna to help the Alidosi of Imola, but he was defeated and the city stormed on February 14, 1424. 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. The Florentine army, this time commanded by Carlo Malatesta, was again defeated, at the Battle of Zagonara in July; Carlo, taken prisoner, was freed by Visconti and joined him too. Florence thus hired Niccolò Piccinino and Oddo da Montone, but the two were also beaten in Val di Lamone. Oddo was killed but Piccinino was able to convince Manfredi to declare war against Visconti.
After the failure in Romagna, Florence tried to defy the Visconti from the Ligurian side, by allying with the Aragonese of Naples. However, both a fleet of 24 Aragonese galleys sent to Genoa to move it to fight against the Milanese, and a land army, were unsuccessful. In the meantime, Piccinino and the other condottiero Francesco Sforza had been hired by Visconti, who also sent an army to invade Tuscany under Guido Torello. He subsequently defeated the Florentine army at Anghiari and Faggiuola.