The heavy tank IS-1 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. It was mainly a breakthrough tank, firing a heavy high-explosive shell that was useful against entrenchments and bunkers. The IS-1 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. The IS-85 (Object 237) prototype was initially accepted for production as the IS-1 heavy tank and first deliveries were made in October 1943. Production ended in January 1944.