The IS-2 entered combat in World War II during the first months of 1944. The Soviets produced significant numbers of the type (close to 4,000), and deployed them with little success against the most advanced German designs of the time, notably the Tiger I, Tiger II, and Panther, as well as against Elefant tank destroyers. The IS2 was best used for bunker assault using its high-explosive ammunition, as its reload rate, just 2 rounds per minute, made it ineffective as a tank destroyer. The IS-3 saw service on the Chinese-Soviet border, the Soviet invasion of Hungary, the Prague Spring and on both sides of the Six-Day War. However the mobility and firepower of medium-tanks and the evolution of the main battle tank rendered heavy tanks obsolete.