Drafted into the Russian army in 1915, he joined the Red Army in 1918 and became a noted cavalry officer; during the Russian Civil War he also met and became a friend of Stalin, which was to help protect him in subsequent decades. In early 1940 he was made commander of Soviet troops fighting Finland, reversing the disastrous early Soviet conduct of that war and forcing the Finns to sue for peace in March 1940. Appointed Peoples Commissar for Defense, he started a modernisation and mechanisation programme for the Red Army. In June 1941 Germany attacked the Soviet Union and Timoshenko was placed in charge of the central front; he is credited with preserving his forces to successfully defend Moscow in late 1941. In May 1942 he led a Soviet assault on Kharkov which had limited success, and he was unable to prevent the German advance on Stalingrad in the summer and autumn of 1942.
He was replaced by Marshal Georgi Zhukov as the prime front line commander, but was subsequently given honorific appointments.