The Soviet missile that prompted NATO’s dual-track decision was the SS-20 Saber. The missile’s Soviet designation was RSD-10 Pioneer; SS-20 Saber was its NATO reporting name.
The SS-20 was a road-mobile, intermediate-range ballistic missile first deployed by the Soviet Union in the 1970s. Unlike many older systems, it could be moved and concealed, improving its survivability. Each missile carried three independently targetable nuclear warheads and could threaten targets across much of Europe and Asia.
NATO responded on December 12, 1979, with a two-track policy. The alliance offered arms-control negotiations while also planning to deploy Pershing II ballistic missiles and ground-launched cruise missiles in Western Europe if negotiations failed. The resulting Euromissile crisis eventually contributed to the 1987 Intermediate-Range Nuclear Forces Treaty, which eliminated the SS-20 and its principal NATO counterparts. SS-20 Saber is therefore the standard answer, even though RSD-10 Pioneer is the Soviet designation.