 Complete the following phrase:  Doumercs 3rd Heavy Cavalry Division was detached from the corps to operate on the northern front where it was in action at the First Battle of Polotsk from 16 to 18 August 1812. Engaged in the action were the 4th and 7th Cuirassier Regiments, each with four squadrons. During the battle, a Russian attack led by some Russian Guard cavalry squadrons pierced the French lines. General Laurent Gouvion Saint-Cyr personally ordered the 3rd Swiss Regiment to attack while General Frédéric de Berckheim led the 4th Cuirassiers in a cavalry charge. These thrusts turned back the Russian assault. Three cuirassier regiments from Doumercs division fought in the 