The Iosif Stalin tanks (or IS tank, also known as the Joseph Stalin tank), was a series of heavy tanks developed by the Soviet Union during World War II and first used in the Kursk area in September 1943. The tanks in the series are also sometimes called JS or ИС tanks.
The heavy tank was designed with thick armour to counter the German 88 mm guns, and carried a main gun that was capable of defeating the German Tiger and Panther tanks at ranges where it was immune to their return fire. It was mainly a breakthrough tank, firing a heavy high-explosive shell that was useful against entrenchments and bunkers. The IS-2 was put into service in April 1944, and was used as a spearhead in the Battle of Berlin by the Red Army in the final stage of the war. In game we have five IS variants: IS-1, IS-100, IS-2, IS-3 and IS-3/D-30.
The IS-100 was a prototype that was developed and eventually became the IS-2. It was armed with a 100mm gun. It went into trials against the IS-122, later the IS-2, which was armed with a 122 mm gun. Though the IS-100, against flat and 30 degree target plates, had better anti-armour capabilities, the latter was chosen due to better all-around performance and better penetration against sloped armour ,such as the 55 degree glacis of the Panther, and thus the IS-122 began production!
The White Cross on the roof and white stripes on the Soviet tanks on the end of the war, was to differentiate their vehicles from the Western Allies. Only in Berlin the stripes never be used. Later that war, vehicles in Berlin are be painted with a white triangle, after the Germans use the white stripes on their tanks to escape Berlin via the West. So that the Western troops could still distinguish between German and Soviet tanks.