When Filippo Maria Visconti, Duke of Milan, died on 13 August 1447, the city was thrown into confusion by his unexpected demise and the speed with which claimants to his title acted.

The claimants to the throne of the Duchy of Milan were: The two most prominent candidates supported by the Milanese population were however Alfonso of Aragon and Francesco Sforza.

The Bracceschi family, supporters of the King of Aragon, seized the city on the 13th, the night of Duke Filippo Maria's death, forcing the captains to swear allegiance to Alfonso.

[4] On the morning of the 14th, Republicans stirred the populace to rise against the Bracceschi, under the leadership of Antonio Trivulzio, Giorgio Lampugnano, Innocenzo Cotta, and Teodoro Bossi (members of the College of Jurisprudence).

[8][9] The idea of a radical renewal of liberties in the cities did not suit the powers of North Italy, who had been in league against Visconti territorial gains in a decades-long series of wars interrupted by truces, most recently the Peace of Cremona of 20 November 1441.

Venice was already at war with Milan, and the Republic was struck a sore blow as previously Milanese cities including Pavia, Lodi, and Piacenza defected or declared their independence.

Milan turned to Francesco Sforza, the greatest military leader of his day, offering him the position of Captain-General and the city of Brescia.

[12] Sforza quickly captured independent Pavia when his mother-in-law persuaded the city's ruling condotierro to relinquish it to him, and the Republic grudgingly allowed him to keep it with the title of Count, fearing that the Pavians and their large arsenal might instead offer themselves to Venice if they refused.

[13] Sforza promised the Pavians no new taxes, respect for the old laws, payment for the officials he retained, and the repair of the city bridges and walls.

The city was also in a strategic location on the Po River, situated where it could block the Venetians from coming to the relief of the beleaguered Piacenza by water.

Two of the Ghibellines who helped to found the Republic, namely Lampugnano and Bossi, stirred the populace against the Guelphs in a massive demonstration before the Court of Arengo.

This victory was celebrated in Milan, but the leaders of the Republic still feared Sforza, and sent him off to occupy himself with the Ghiarad'adda region while Lodi and Caravaggio were besieged, hoping that their fall would end the war.

The siege remained unbroken throughout July and until 15 September, when Attendolo launched a surprise-attack on Sforza, so rapidly that he did not even have time to buckle on his armor.

What might have been a rout Sforza turned into a great victory as he set the example for his troops on the front line and utterly defeated the Venetian army, capturing three generals.

[19][20] Caravaggio fell, and despite rejoicing in Milan, little gratitude was given by the government to the architect of the victory, and the Republic sent him on to Lodi, determined to end the war.

[26] Gonzaga kept this knowledge secret, knowing that Lampugnano and Bossi were two of the most influential citizens since the formation of the Republic, but with the support of the vengeful Guelph Captains and Defenders conspired to have them slain.

Louis of Savoy invaded in spring of 1449, and Sforza sent Colleoni (who had earlier defected) to defeat him at Borgomanero, leading to an uneasy peace.

[29] Sforza also faced treachery within his own ranks, added with the fact that he rashly accepted the defection of his great enemies the Piccinino brothers, who, upon gaining access to Monza, promptly returned it to Milan.

[32] Gonzaga, however, whose friend Galeotto Toscano was killed in the uprising, departed Milan for Crema, hoping to make peace with Sforza.

After reinforcing his peace with Savoy with the concession of a few unimportant castles, he defeated the Venetians under his rival Sigismondo Malatesta and continued the siege.

[38] Starvation and suffering were rampant in Milan, and the Venetians had sent an ambassador, Leonardo Venieri, to negotiate the city's surrender and help them defeat Sforza.

At last, Gaspare da Vimercate and several members of the Stampa family engineered a coup on 24 February 1450, which resulted in the murder of Venieri on the stairs of Palazzo Reale.

The continued war was finally concluded by the peace of Lodi in 1454 with the House of Sforza established as the rulers of the Duchy of Milan[45] During its three-year existence, the Ambrosian Republic won two major battles.

For there are, among the nobles, so many exalted personages that the laws do not suffice to repress them, and they must needs be kept under by a living voice and a royal power.The leading magistrates of the city, the capitani e defensori ("Captains and Defenders"), were in charge of the government, elected every six months beginning in August 1447.