Armed with a powerful 76.2mm main gun and equipped with 75mm frontal armor, the KV-1 tank was an unpleasant surprise for the German army when they invaded Russia in the summer of 1941. Impervious to nearly all anti-tank guns, the KV-1 proved to be a formidable opponent, but the introduction of newer, more powerful German tanks lessened its advantage as the war went on. In response, additional armor plates were bolted onto the hull and turret to increase protection, resulting in the KV-1B variant.