 Complete the following phrase:  First he joined the Cossack detachment of Novocherkassk and participated in the battle for Rostov in NovemberDecember 1917. Then he became Chief of Staff of the Don Cossack Army, under command of General Anatoly Nazarov. Later he was also Chief of Staff under General Pyotr Popov 25 April  18 May 1918, when he took part in the 