Nuclear triad

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In nuclear strategy, the nuclear triad refers to the three tiers of a country's nuclear arsenal, traditionally composed of strategic bombers (carrier-based or land-based; armed with bombs or missiles), land-based missiles (MRBMs or ICBMs), and ballistic missile submarines (SSBNs). To be considered a part of a nuclear triad, weapons must have a first- or second-strike function, lest they be considered a tactical nuclear weapon.

The possession of a nuclear triad significantly reduces the possibility that an enemy could destroy all of a country's nuclear forces in a first strike attack, ensuring a credible threat of a second strike. This increases a nation's nuclear deterrence.

The triad also gives the commander the possibility to use different types of weapons for the appropriate strike:

ICBMs allow for a long-range strike launched from a controlled or friendly environment. If launched from a fixed position, such as a missile silo, they are vulnerable to a first strike, though their interception once aloft is substantially difficult.
SLBMs, launched from submarines, allow for a greater chance of survival from a first strike, giving the commander a second-strike capability. Their limited range, however, means that submarines must often be closer to its target than desirable, allowing for potential detection and tracking by an enemy attack submarine or a SOSUS-like system.
Strategic bombers have greater flexibility in their deployment and weaponry. They can serve as both a first- and second-strike weapon. A stealth B-2 Spirit bomber armed with stealth AGM-129 ACM missiles, for example, could be classified as a first-strike weapon. A number of conventional bombers kept at fail-safe points would constitute a second-strike weapon.

Other potential delivery systems include orbital weapons or spacecraft for purposes of orbital bombardment using nuclear devices. However, space-based weapon systems used for weapons of mass destruction have been banned under the Outer Space Treaty and the SALT II treaty.

Not all nuclear powers have triads, nor are all triads configured the same way. For example:

France- The Force de frappe possessed, in the past, a "traditional" nuclear triad, though it currently possesses only sea-based and air-based components.
United Kingdom- The Royal Navy through its Vanguard class submarines is the only nuclear deterrent of the United Kingdom.
Israel- In addition to its Jericho MRBMs and aircraft, Israel is unique in reportedly using submarine-launched cruise missiles as part of its deterrent (as opposed to ballistic missiles).
India & Pakistan- Both countries lack a dedicated naval component to their nuclear deterrent. Existing or planned cruise missiles, however, are thought to be adaptable for this purpose.