form of second-strike capability by fielding continual patrols of strategic nuclear bombers, with a large number of planes always in the air, on their way to or from fail-safe points close to the borders of the Soviet Union. This meant the United States could still retaliate, even after a devastating first-strike attack. The tactic was expensive and problematic because of the high cost of keeping enough planes in the air at all times and the possibility they would be shot down by Soviet anti-aircraft missiles before reaching their targets. In addition, as the idea of a missile gap existing between the US and the Soviet Union developed, there was increasing priority being given to ICBMs over bombers. It was only with the advent of ballistic missile submarines, starting with the George Washington class in 1959, that a genuine survivable nuclear force became possible and a retaliatory second strike capability guaranteed. The deployment of fleets of ballistic missile submarines established a guaranteed second-strike capability because of their stealth and by the number fielded by each Cold War adversary—it was highly unlikely that all of them could be targeted and preemptively destroyed (in contrast to, for example, a missile silo with a fixed location that could be targeted during a first strike). Given their long-range, high survivability and ability to carry many medium- and long-range nuclear missiles, submarines were credible and effective means for full-scale retaliation even after a massive first strike. This deterrence strategy and the program have continued into the 21st century, with nuclear submarines carrying Trident II ballistic missiles as one leg of the US strategic nuclear deterrent and as the sole deterrent of the United Kingdom. The other elements of the US deterrent are intercontinental ballistic missiles (ICBMs) on alert in the continental United States, and nuclear-capable bombers. Ballistic missile submarines are also operated by the navies of China, France, India, and Russia. The US Department of Defense anticipates a continued need for a sea-based strategic nuclear force. The first of the current Ohio-class SSBNs are expected to be retired by 2029, meaning that a replacement platform must already be seaworthy by that time. A replacement may cost over $4 billion per unit compared to the USS Ohios $2 billion. The USN's follow-on class of SSBN will be the Columbia class, which began construction in 2021 and enter service in 2031. ABMs threaten MAD In the 1960s both