Nuclear submarines are sea vehicles that are capable of remaining underwater for months at a time. This was possible because of the transition into nuclear, rather than diesel, generators. Typically equipped with both nuclear and anti-submarine missiles, they presented a new threat to those nations engaging in nuclear deterrence. Since these mobile, virtually invisible ships could close to within a few dozen miles of a target, the reaction time to a nuclear attack could be reduced to minutes. Any nation who relied solely on land based missile launch systems stood a very good chance of being crippled before any opportunity to retaliate. This imbalance was righted however, as all announced nuclear powers soon had their own fleet of nuclear attack submarines. The difficulty in detection of nuclear submarines means that only other nuclear submarines had a viable chance at stopping one.