This is a variation of the "The Sum of All Fears" scenario, where, instead of the lost Israeli warhead being buried by a Druze farmer, it's immediately found by the Syrians and turned over to the Soviets, who have to keep everything completely quiet for this to work. After taking apart the warhead to study it, the American-made fissile material is repackaged into a new tactical warhead that is then smuggled back to the US for Spetsnaz use. At the start of a hypothetical conventional WWIII in the 80s, the warhead is detonated as a nuclear truck bomb near the Pentagon in Washington DC. Nuclear retaliation by the US would likely seem inevitable, but I think there are three important points that may militate against that. 1. This is the only nuclear explosion. There is no large-scale Soviet nuclear attack and Soviet nuclear forces are not visibly preparing to attack. Edit: this is the only nuclear demolitions executed by the Spetsnaz. 2. No delivery method is observed. Even with only a few days of war-warning, air defenses around the NCR would be capable of comprehensive surveillance, but no incoming missile or bomber would be observed in this scenario. The ground burst could indicate a ground-based delivery method, which would not be outright proof of Soviet involvement. 3. Analysis of the fissile material would indicate US, not Soviet, origins. Combined with the "truck bomb" style delivery, that could indicate a third party terrorist attack, though the timing would be suspect. Attacking on the second or third day of the war might reduce that suspicion. Without nuclear war obviously imminent and with the Soviets possibly able to blame the attack on some American domestic group, I would doubt the existence of any sort of NATO consensus to escalate towards outright nuclear war.