SenorBeef nailed it.
In the event Russia triggers a conflict with NATO, whether inadvertently or deliberately, events will become hostage to the ebb and flow of the war on the ground. Once that war is going badly enough for either side, the temptation to tip the scales with a nuke or two will become all but irresistible.
The Russians were always more prone to this than was NATO, even back in the middle Cold War years when NATO’s public position was that it needed nukes to offset the Soviet’s / Warsaw Pact’s clear numerical superiority, presumed element of surprise/ tactical initiative, and presumed qualitative near parity.
The Soviets then and Russians now have never considered nuclear war “unthinkable”. That word simply isn’t in their doctrinal lexicon. Nukes are just another weapon on the spectrum of weapons from smaller to larger. Nuclear wars, especially but not exclusively limited nuclear wars, are fully fightable and fully winnable, period amen. So says their doctrine today and has for decades.
Clear thinkers on both sides should appreciate the difference between how they themselves perceive nukes and how their opposition does. This thinking applies to all aspects of politics, strategy, doctrine, and tactics, but becomes especially acute at the high end of the destructive curve. Truly understanding your enemy is essential to correctly predicting their next moves. Failing to understand your enemy, assuming they’re a mirror of yourself, or assuming they’ll react like an NPC goon so you can blast them more easily, is a direct route to your own defeat.
As such, smart Russian leaders understand that NATO sees a strong firebreak at nuclear first use, and will probably react disproportionately to Russia crossing that break. Conversely, smart NATO leaders should expect the Russians not to think in terms of a firebreak at all. Once a nuclear-armed Russian unit starts losing severely it will naturally use nuclear fires to preserve itself.
Whether leaders on either side will be blinded by their own side’s self-interested narratives is always an open question. E.g. The Japanese high command in 1940 firmly believed that the US was so in love with big cars and easy living that they’d cede the Pacific to Japan if sharply pushed. Seems ludicrous to us now, but within their inner circle that was in fact the group-think consensus.
What do you think is the group-think consensus in Putin’s inner circle today about the robustness of NATO & more specifically US resolve in the face of a stiff punch in the nose? Under Obama, under Clinton, or under Trump? What of May, Hollande, and Merkel?
Finally, a war always ends in one of two ways: a stalemate/negotiated settlement at something near the pre-war borders with both belligerent governments surviving (See Korea in the 1950s and Gulf War I) or as the comprehensive replacement of one belligerent’s regime (See Nazi Germany in 1945 or Gulf War II).
No country that has nuclear weapons will submit to Option 2 without using at least some of them. Anyone fighting a currently-conventional war against a nuclear power knows this. Which means in a very real sense that no nuclear power can be defeated without the victor being willing to absorb some nuclear strikes. At most the losing side can be frustrated in its goal of territorial growth. But it will live on to regroup and perhaps fight another day.
And this logic is precisely what drove the regimes in Pakistan and North Korea to obtain nukes, and has been (still is?) driving Iran in the same direction.