 Complete the following phrase:  Advancing in tandem with the Southeastern Front from November, the Southern Front fought in the Kharkov operation between 24 November and 12 December, capturing Stary Oskol, Novy Oskol, Sumy, Biryuch, Volchansk, Valuyki, Kupyansk, Belgorod, Kharkov, and Pavlovsk. In the Donbass operation, it crossed the Seversky Donets, capturing Izyum, Slavyansk, Bakhmut, Debaltsevo, Gorlovka, Ilovaysk, Konstantinovka, Lugansk, and Mariupol. The front pushed the Armed Forces of South Russia back to the Black Sea and North Caucasus by early 1920, and moved its headquarters forward to Kursk in early January. Its troops inflicted heavy losses on the Armed Forces of South Russia, and fought in the 