The forces of Lannoy and of Gouffier clash near the Sesia River, on April 30, when the Spanish-Imperial troops intercept the French.

Bonnivet's forces (where many of the Swiss mercenaries abandon the French army) suffer a decisive defeat by Lannoy's forces.

Pierre Terrail, Lord of Bayard, after allaying a revolt at Genoa, and striving with the greatest assiduity to check a pestilence in Dauphiné, Bayard had been sent into Italy with Admiral Bonnivet, who, being defeated at Robecco and wounded in a combat during his retreat, implores Bayard to assume command and save the army.

He repulsed the foremost pursuers, but in guarding the rear at the passage of the river Sesia between the towns of Romagnano Sesia and Gattinara, is mortally wounded by an arquebus ball, another demonstration of the power of the Spanish arquebusiers against more traditional troops.

He dies in the midst of the enemy, attended by Pescara, the Spanish commander, and by his old comrade, Charles, duc de Bourbon, who is now fighting on the opposite side.

Charles is reported to have said “Ah! Monsieur de Bayard... I am very sad to see you in this state; you who were such a virtuous knight!”

Bayard answered, “Sir, there is no need to pity me. I die as a man of honor ought, doing my duty; but I pity you, because you are fighting against your king, your country, and your oath.”

His body is restored to his friends and will be interred at Saint-Martin-d'Hères.

Finally, the French army retreats into the Alps in disarray.