Complex sentence: The T-26 was a light tank assigned to infantry units and thus fulfilled the infantry tank role, but it had the relatively thin armour of a light tank, but with a potent 45 mm gun.
Split into simpler sentences: The T-26 was a light tank assigned to infantry units and thus fulfilled the infantry tank role. It had the relatively thin armour of a light tank, but with a potent 45 mm gun.