Chevaline (/ˈʃɛvəliːn/) was a system to improve the penetrability of the warheads used by the British Polaris nuclear weapons system. Devised as an answer to the improved Soviet anti-ballistic missile defences around Moscow, the system increased the probability that at least one warhead would penetrate Moscow's anti-ballistic missile (ABM) defences, something which the Royal Navy's earlier UGM-27 Polaris re-entry vehicles (RVs) were thought to be unlikely to do.
Chevaline used a variety of penetration aids and decoys to offer so many indistinguishable targets that an opposing ABM system would be overwhelmed attempting to deal with them all, ensuring that enough warheads would get through an ABM defence to be a reasonable deterrent to a first strike. The project was highly secret, and survived in secrecy through four different governments before being revealed in 1980.
The system was in service from 1982 to 1996, when the Polaris A3T missiles it was fitted to were replaced with the Trident D5.