When Soviet infantry units found themselves facing German tanks on the offensive, the last line of defence was typically a wall of anti-tank guns. Either they stopped the Blitzkrieg, or whole divisions would be annihilated.
EFFECT:
Whenever an enemy Light Tank ⚡ or Medium Tank ⚡ fires at this cannon or a vehicle equipped with it, they may Counterattack (0 AP) with Reaction Fire (1 AP).