Editing Mutual assured destruction (section)

===Retaliation capability (second strike)===
{{see also|Second strike}}
[[File:Robert McNamara official portrait.jpg|thumb|120px|left|[[Robert McNamara]]]]
The strategy of MAD was fully declared in the early 1960s, primarily by [[United States Secretary of Defense]] [[Robert McNamara]]. In McNamara's formulation, there was the very real danger that a nation with nuclear weapons could attempt to eliminate another nation's retaliatory forces with a surprise, devastating first strike and theoretically "win" a nuclear war relatively unharmed. The true second-strike capability could be achieved only when a nation had a ''guaranteed'' ability to fully retaliate after a first-strike attack.<ref name=":0" />

The United States had achieved an early 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 [[Surface-to-air missile|anti-aircraft missile]]s 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 [[ICBM]]s over bombers.

[[File:USS George Washington (SSBN-598) underway at sea, circa in the 1970s.jpg|thumb|210px|right|The [[USS George Washington (SSBN-598)|USS ''George Washington'' (SSBN-598)]], the lead ship of the [[US Navy]]'s first class of [[Ballistic missile|Fleet Ballistic Missile Submarines, Nuclear (SSBN)]]]]
It was only with the advent of [[nuclear submarine|nuclear-powered]] [[ballistic missile submarine]]s, starting with the [[George Washington class submarine|'' George Washington'' class]] in 1959, that a genuine [[Survivability|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.<ref>{{cite web | url=https://www.acq.osd.mil/ncbdp/nm/NMHB2020rev/chapters/chapter3.html | title=NMHB 2020 &#91;Revised&#93; }}</ref>

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 deterrence|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 [[United States Department of Defense|US Department of Defense]] anticipates a continued need for a [[Nuclear triad|sea-based strategic nuclear force]].{{cn|date=April 2024}} The first of the current [[Ohio-class submarine|''Ohio''-class SSBNs]] are expected to be retired by 2029,{{cn|date=April 2024}} 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 ''Ohio''{{'}}s $2 billion.<ref name="Frost">
 {{cite news
 |url = http://www.dailypress.com/news/dp-local_subs_0424apr24,0,5810806.story
 |title = Newport News contract awarded
 |newspaper = [[Daily Press (Virginia)|Daily Press]]
 |last = Frost
 |first = Peter
 |access-date = 2011-09-27
 |url-access=subscription 
 |url-status = dead
 |archive-url = https://web.archive.org/web/20090426041620/http://www.dailypress.com/news/dp-local_subs_0424apr24,0,5810806.story
 |archive-date = 2009-04-26
}}</ref> The USN's follow-on class of SSBN will be the [[Columbia-class submarine|Columbia]] class, which began construction in 2021 and enter service in 2031.<ref>{{cite web |title=Navy Columbia(SSBN-826) Class Ballistic Missile Submarine Program: Background and Issues for Congress, Updated June 24, 2019 |url=https://fas.org/sgp/crs/weapons/R41129.pdf |archive-url=https://web.archive.org/web/20110504225457/http://www.fas.org/sgp/crs/weapons/R41129.pdf |archive-date=2011-05-04 |url-status=live |website=fas.org |publisher=Congressional Research Service |access-date=July 17, 2019}}</ref>