The French advanced through the Cadibona Pass to defeat the isolated right wing of the Habsburg army, commanded by Feldmarschal-Leutnant (FML) Eugène-Guillaume Argenteau, at the Battle of Montenotte on 12 April.

On 13 April, General of Division Pierre Augereau attacked FML Giovanni di Provera's weak Austrian Auxiliary Corps east of Millesimo and defeated him.

To cover the withdrawal of his command, Provera retreated to Cosseria Castle with part of the Hasbsburg Gyulai Freikorps and two grenadier companies of the Strassoldo Infantry Regiment # 27.

After leading the last attack of the day, Colonel Barthélemy Joubert wrote, Nothing more terrible could be imagined than the assault, where I was wounded passing through a loophole; my carabiniers held me up in the air, with one hand I grasped the top of the wall.

There was, in fact, no real battle at Millesimo itself, but rather a confused action on 13 April, in which a number of small enemy units were driven back, followed by a short but very costly siege of the castle of Cosseria, which was defended by only about a thousand Austrians and Piedmontese under Provera and Del Carretto.

It was only after the defenders had been forced to surrender the castle on 14 April, due to lack of ammunition, food and water, that the French advance inland could continue in safety.