The battle of Agnadello (14 May 1509) was the only major battle of the War of the League of Cambrai, and was a Venetian defeat that caused the Republic to temporarily abandon many of its mainland possessions.
The League of Cambrai was an alliance between Louis XII of France, the Emperor Maximilian, Ferdinand II of Aragon and Pope Julius II, aimed at dismantling the mainland empire of Venice. Each of the participants had their own claim against part of the Venetian empire (of different levels of validity), and if all of the allies had coordinated their attacks then the Republic might have been in real trouble. Instead the allies operated independently. Maximilian attacked before the League had been agreed, but suffered an embarrassing defeat at Cadore (2 March 1508).
The most serious threat to the Republic came from the French, who had recently conquered Milan. At that time they had been allied with Venice, and the Republic had been given all Milanese territory east of the River Adda. Louis XII commanded the French army in person, while amongst the French troops was Charles IV, Duke of Alencon, later disgraced at Pavia.
When the war began the Venetian army on the Adda was commanded by Bartolomeo Alviano and Nicolo Orsini, Count of Pitigliano. Alviano was an able commander who wanted to take an aggressive stance and attack across the Adda, but Orsini was more conservative and blocked this plan.
The French were handed an advantage when Treviglio, east of the Adda (almost due east of Milan) surrendered to them. Pitigliano decided to punish the town and moved to attack it. Treviglio was recaptured and burnt, but this move allowed the main French army, under Louis XII, to make an unopposed crossing of the Adda at Cassano, west of Treviglio.
Louis's next move was to turn south towards Pandino. This threatened to cut communications between the Venetian army and Crema and Cremona to the south-east. The Venetians were forced to rush south in an attempt to block this move. Their light cavalry was sent to occupy Pandino and Palazzo, while the main army moved along higher ground further east of the river. The two armies thus ended up moving parallel, with the French on a lower road nearer the Adda.
The Venetian column was soon spread out along four miles of roads, with Alviano leading the front half of the army. The French took advantage of this to launch a surprise attack on the Venetian advance guard. The Venetians had the advantage of higher ground and a good defensive position amongst vineyards, and were able to hold the French off for some time.
Alviano wasn't present when the attack began, but soon arrived with some of the Venetian heavy cavalry and infantry. These were the only Venetian reinforcements to reach the fighting, while the French continued to feed their main force into the battle. Pitigliano refused to come to the aid of the advance guard, effectively dooming Alviano's men. The Venetian levies suffered heavy losses during the battle, and Alviano's own infantry unit, from Brisighella, was almost annihilated. The advance guard was eventually defeated and Alviano himself was captured.
Pitigliano managed to retreat with the main body of men at arms, but most of the rest of the army broke and fled. The French captured 36 guns, a sign of the size of their victory.
In the aftermath of this defeat the Republic rapidly withdrew from Bergamo, Brescia, Crema and Cremona, all of which were taken by the French. They evacuated their possessions in the Romagna, which were taken over by the Pope. Verona, Vicenza and Padua were allowed to surrender to representatives of the Emperor. Their recent conquests east of Venice were abandoned, as were the coastal towns in Apulia they had taken from Naples.
The Allies missed the chance to inflict a really heavy defeat on Venice. The French didn't press them, and Maximilian didn't move until June. By this time the Republic had regained much of its confidence and had reoccupied Padua. Maximilian became bogged down in a siege of Padua (8 August-30 September 1509), where he was supported by a small French contingent. The failure of this siege marked the end of any significant fighting in the war, and soon afterwards the League of Cambrai began to fall apart. In February 1510 Pope Julius agreed a treaty with Venice, the first step in the formation of the anti-French Holy League. During the resulting War of the Holy League (1510-1514) the Venetians were able to regain many of their losses of 1509, although the Republic never quite regained the position of power it had held before the battle.