Hello. I'd like to chip in some information in this conversation, although it has periphery relevance to Cold Waters.
The most prominent sources I've come across (there may be others) describing a Soviet European offensive with heavy NBC participation would be Wilbur Gray's article arguing that the Soviets restored to a nuclear-heavy war plan in the face of a supposed NATO conventional superiority, (rather sensationalist) news articles about the "Seven Days to the Rhine war plan", and some academic articles arguing that DDR on Warsaw Pact military exercises was evidence that the a nuclear first-strike war plan. I found the 1st to be deficient in its understanding of Soviet military art, the second to be sensationalism, and the third (as pointed out in this
page) a "confusion between first-strike and "preemption"; the latter meant "attempting to strike an enemy that is preparing to a launch a nuclear strike before he is able to launch."
To paraphrase the readings on Soviet military art from the Soviet Army Studies Office (SASO), the Soviet Studies Research Centre (SSRC), David Glantz's articles, and the Voroshilov Lectures book series, the Soviet military adopted a "nuclear only" option during the 1960s under Khrushchev. Following his downfall, the Soviets expected waging either full-scale nuclear war or large-scale conventional war in a "nuclear-scared context", albeit for a clear preference for the conventional. During the late 1970s and the early 1980s, military reforms under Marshal Ogarkov refined the Soviet military towards conventional mechanized warfare. By the end of the 1980s, Soviet military theorists believed that next-gen Western systems (SADARM, the E-8 JSTARS plane, the canceled "Assault Breaker" etc) and "weapons of new physical principles" were going to make traditional mechanized forces in NATO and Warsaw Pact armies obsolete.
IMO, the Soviet would only use nuclear weapons in (1) in the face of a NATO/Chinese offensive that would threaten the Soviet homeland, and (2) if NATO was going to launch a massive and effective tactical-theater nuclear strike (the Voroshilov book pointed out that the Soviets might have tolerated limited NATO nuclear escalation to show resolve, albeit they doubted it would say limited for long). On the contrary, instead of "restoring momentum" to a conventional attack, nukes would generate large zones of destruction that would impede rapid forward movement--think about how nukes would block roads in forested and close terrain, and create unpredictability for military planners and the state leadership. As for chemical weapons, the Sovietologist Chris Donnelly argued that they would slow down the rate of advance and NATO might escalate with nukes, meaning that they (like nukes) was more of a fear and deterrent weapon.
I'd argue that Ralph Peters' 1989 novel
Red Army represents the most "believable" NATO-WP war scenario in fiction--it's quite clear he was reading similar source material as SASO's and SSRC's Sovietologists. However, the story ends on the third day of the war with a unilateral West German ceasefire...