You are very wrong. Stalin not only did serve in the army, but even in cavalry during the Polish-Soviet war. Read Norman Davies' White Eagle Red Star. He was directly responsible for not moving with his troops toward Warsaw, he was trying to seize Lwów instead, against orders. Of course he failed to seize Lwów and his insubordination was a key to the successful Polish offensive south of Warsaw.
No, dear Musashi! I am always sure in what I am writing, so I am very right
. Stalin was responsible for many military things during the Russian Civil and Russian-Polish wars as he was the member of war councils of several fronts as bolshevist (for example, he organized the defense of Tsaritsyn (later Stalingrad) against Krasnov's troops in 1918-1919, served as chief representative in the defense of Petrograd in 1919, planned the operations against Denikin, was the chairman of the war council of Ukrainian labor army).
During the Polish-Soviet war 1920 Stalin was the member of revolutionary-military council of Soviet South-West front, he was not a commander of any division or army. Stalin was proud that his "military genius" made the Kiev offensive, Volyn and Rovno operations 1920 very successful for Red troops and caused the retreat of Polish armies, but in reality those operations were planned by the commander of South-West front A. Egorov and Stalin only made the operations more difficult because of incompetence. As for Lvov operation 1920 - "military genius" of Stalin caused its failure as he disturbed the connections between West and South-West fronts, gave the order (despite the protests of other commanders of South-West front) for the 1st cavalry army to take Lviv without taking into consideration the facts that 1st cavalry army was of incomplete structure because of previous heavy combats for Brody.
Stalin didn't have any military education and never served in army as professional, he served in front HQs as the representative of bolshevist party.