Stalin's invasion plan mainly involved ground forces. Additionally, the northern Russian waters are icy, providing a major natural obstacle to any sort of surface navy. As the result, the Soviet Navy was seriously under equipped. But as a sea force was quickly determined to be necessary, a ship that could either break through ice or dive underneath it was required. As submarines trade off increased construction costs for extra protection from air attack and long range detection, the Soviets chose to develop the Victor class submarine.