From the campaign:
1A Games’s Stalingrad expansion for the World War Two tactical board game series Tide of Iron (TOI) introduces players to the unprecedented, vicious street fighting that came to symbolize the brutality of what the Soviet Union called its Great Patriotic War.
You must have the TOI starter set in order to play the Stalingrad expansion. Don't have the starter set yet? You can get the new "Next Wave" edition with special quick-start scenarios and some new, additional items (explained below) as a reward at the REGIMENT or CORPS levels.