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.
Besides the loss of support and defensive locations, the drop in revenue also caused a brief financial crisis resolved by the imposition of new taxes.
Venice, now occupying Lodi and Piacenza, refused to listen to Milan's pleas for peace.
Milan turned to Francesco Sforza, the greatest military leader of his day, offering him the position of Captain-General and the city of Brescia.
Although he wished – and indeed, planned – to succeed his father-in-law as Duke, he decided to accept the position and promised rewards.
 A draft was declared in Milan on 13 September.

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.
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.
He kept these promises faithfully and so won over the people of Pavia, establishing his rule.
Pavia was previously almost a second capital to Milan, and gave Sforza his own seat of power.
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.
Returning to the siege of Piacenza and finding the city not likely to crumble by starvation, he resolved to storm it.
Employing cannon in an almost unheard-of manner, he opened up a breach in Piacenza's walls and sacked the city.
The news of Piacenza's sack was greeted by three days' rejoicing in Milan.
Meanwhile, Milanese Captain Bartolomeo Colleoni captured Tortona which had previously proclaimed Sforza as its lord, stealing it from the now dangerously powerful Sforza.
 This caused even greater rejoicing, for the Milanese feared Sforza.

In January 1448, the terms of the previous Captains and Defenders ended, and in the following election the Guelphs rose to political prominence.
Being especially adverse to Sforza, they entered peace negotiations with Venice, which, also threatened by Sforza's growing power, was willing to come to terms, albeit dictated by herself.
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.
The Council of Nine Hundred was forced to abandon its plans for peace, and Sforza was given the go-ahead for his next campaign to seize the Adda River.
However Sforza's plans were voted down by the Republic in favor of more traditional tactics, namely besieging Lodi directly.
While this happened, the Venetian fleet under Andrea Quirini assaulted the bridge of Cremona.
However, Sforza's wife Bianca was there, and led the defense until her husband relieved the city.
The Venetian fleet withdrew and entrenched while it waited for the arrival of the Venetian army.
Sforza trapped the fleet behind its defenses and, with his unorthodox use of artillery, utterly destroyed or captured every one of the seventy ships.
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.

Ultimately both Sforza's Milanese forces and the Venetian army under Micheletto Attendolo met at Caravaggio.
The siege remained unbroken throughout July and until 15 September, when Attendolo launched a surprise-attack on Sforza, so rapidly
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. Besides the loss of support and defensive locations, the drop in revenue also caused a brief financial crisis resolved by the imposition of new taxes. Venice, now occupying Lodi and Piacenza, refused to listen to Milan's pleas for peace. Milan turned to Francesco Sforza, the greatest military leader of his day, offering him the position of Captain-General and the city of Brescia. Although he wished – and indeed, planned – to succeed his father-in-law as Duke, he decided to accept the position and promised rewards.  A draft was declared in Milan on 13 September.
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. 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. He kept these promises faithfully and so won over the people of Pavia, establishing his rule. Pavia was previously almost a second capital to Milan, and gave Sforza his own seat of power. 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. Returning to the siege of Piacenza and finding the city not likely to crumble by starvation, he resolved to storm it. Employing cannon in an almost unheard-of manner, he opened up a breach in Piacenza's walls and sacked the city. The news of Piacenza's sack was greeted by three days' rejoicing in Milan. Meanwhile, Milanese Captain Bartolomeo Colleoni captured Tortona which had previously proclaimed Sforza as its lord, stealing it from the now dangerously powerful Sforza.  This caused even greater rejoicing, for the Milanese feared Sforza.
In January 1448, the terms of the previous Captains and Defenders ended, and in the following election the Guelphs rose to political prominence. Being especially adverse to Sforza, they entered peace negotiations with Venice, which, also threatened by Sforza's growing power, was willing to come to terms, albeit dictated by herself. 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. The Council of Nine Hundred was forced to abandon its plans for peace, and Sforza was given the go-ahead for his next campaign to seize the Adda River. However Sforza's plans were voted down by the Republic in favor of more traditional tactics, namely besieging Lodi directly. While this happened, the Venetian fleet under Andrea Quirini assaulted the bridge of Cremona. However, Sforza's wife Bianca was there, and led the defense until her husband relieved the city. The Venetian fleet withdrew and entrenched while it waited for the arrival of the Venetian army. Sforza trapped the fleet behind its defenses and, with his unorthodox use of artillery, utterly destroyed or captured every one of the seventy ships. 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.
Ultimately both Sforza's Milanese forces and the Venetian army under Micheletto Attendolo met at Caravaggio. The siege remained unbroken throughout July and until 15 September, when Attendolo launched a surprise-attack on Sforza, so rapidly
Golden Ambrosian Republic