Nuclear Powered Ballistic missile submarines carrying Submarine Launched Ballistic Missile (SLBM) are the most survivable part of a nuclear triad. While a first strike might wipe out a country’s land-based missiles and nuclear bombers, it’s very difficult to track and take out a ballistic-missile submarine patrolling quietly in the depths of the ocean. Since they can’t be taken out by ‘first strike’ and can retaliate against an enemy with SLBMs, no sane adversary will attack a country which possesses a Nuclear Powered Ballistic missile submarines armed with SLBMs.