On 23 July/4 August 1812 he entered the Moscow militia and participated in the battles at Borodino, Tarutinsk, and Malojaroslavets. The raising of the regiment, which was called after its chief "1st Mounted Cossack Regiment" Count M.A. Dmitriev-Mamonov, went on only sluggishly, though the popular cavalry colonel Prince B.A. Svjatopolk-Tshetvertinsky had been named its commander. The regiment consisted in part of the count's serfs, but in part also of volunteers. The officers belonged to the Moscow court nobility. Among them was Prince P.A. Vyazemsky, who later wrote of these times: