The Nuclear Triad
In strategic nuclear parlance, the Nuclear Triad refers to the three tiers of a countrys nuclear arsenal. Composed of land-based ICBMs, ballistic missile submarines armed with SLBMs, and strategic bombers armed with bombs or cruise missiles.
This reduces the possibility that an enemy could destroy all of a countrys nuclear forces in a first strike attack. This ensures a credible threat of a second strike, and is the cornerstone of the doctrine of Mutual Assured Destruction MAD.