In May 1919, Denisenko was drafted into the Red Army and assigned to the 3rd Lebedinsky Guard Company as a Red Army man before being sent to the reserve battalion of the 8th Army and then the 102nd Rifle Regiment of the 12th Rifle Division. With the division, he fought in operations against Anton Denikin's army on the Don and in the Donbass during 1919. From December 1919, Denisenko fought with the 45th Rifle Division during the Odessa Operation and the Kiev Offensive. From late November 1920, he was a company political officer in the 420th and 397th Rifle Regiments of the 47th Rifle Division. In 1920, Denisenko joined the Communist Party.