In 1919 the leader of the Communistic Party was Vladimir Lenin, not Joseph Stalin as shown in the movie, who gained this position only in 1922.
The leadership of the Armoured Train in May 1919 was held by Leo Trotzky in real life, not by Joseph Stalin as shown in the movie.
Joseph Stalin was never an Admiral on a battle ship.
Unlike in the movie Joseph Stalin was never on the north front and never had any influence on the navy operations.
The support of the White Army by Winston Churchil didn't happen until September 1919, while in the movie it happened in May 1919. Furthermore The Kronstadt attack by the British army was in July 1919, also months it is set in the movie.