Grechkin joined the Red Army in August 1918 and was appointed assistant commissar of the staff of the Don Soviet Republic. He subsequently fought on the Southern Front of the Russian Civil War as a battalion commander, chief of defense of Morshansky Uyezd, and commander of the reserve regiment of the 9th Kuban Army. He fought in battles with the White Army 4th Don Cavalry Corps, the elimination of Sergei Ulagay's landing force, and the suppression of partisans in the Kuban.