Essentially, the US wants Russia to stand down from its signaled intent of possibly using tactical nuclear weapons if NATO’s reportedly 100,000-strong invasion force crosses the Dnieper, which could occur if Russia achieves a military breakthrough.
Polish Foreign Minister Radek Sikorski revealed in his latest interview with The Guardian that “The Americans have told the Russians that if you explode a nuke, even if it doesn’t kill anybody, we will hit all your targets [positions] in Ukraine with conventional weapons, we’ll destroy all of them. I think that’s a credible threat.” If true, and there’s no reason to suspect that he simply made that up, then this amounts to the US playing a dangerous game of nuclear chicken with Russia.
As was explained in this analysis here about why Russia is presently undertaking tactical nuclear weapons exercises, it hopes to deter NATO from a conventional military intervention in Ukraine, barring which it wants to signal that it could resort to these arms if those forces cross the Dnieper. From Russia’s perspective, the reportedly 100,000-strong force that NATO is preparing to invade Ukraine if its “red lines” are crossed could pose a threat to its territorial integrity if they attack its newly unified regions.
So long as they stay on the western side of the Dnieper, then there’d be no reason for Russia to countenance using tactical nuclear weapons, but they could realistically be employed in the event that they cross the river and credibly appear to be approaching that country’s new borders. In that scenario, Russia would have reason to drop them on the invading forces as a last resort out of self-defense to preemptively neutralize this threat in accordance with its nuclear doctrine.
Having brought the reader up to speed about the context within which Sikorski shared the US’ planned response to Russia potentially exploding nukes in Ukraine, it should now be easier to understand why this amounts to a dangerous game of nuclear chicken. Essentially, the US wants Russia to stand down from its signaled intent of possibly using tactical nuclear weapons if NATO’s reportedly 100,000-strong invasion force crosses the Dnieper, which could occur if Russia achieves a military breakthrough.
If this sequence of events unfolds – the front lines collapse, NATO conventionally intervenes in Ukraine, its reportedly 100,000-strong invasion force crosses the Dnieper, Russia drops tactical nukes on them, and then the US hits all of its forces in the newly unified regions – then World War III would break out. There’s no way that Russia would sit back and let the US directly attack any target within its borders since it’ll either respond in a tit-for-tat fashion or jump to the chase by launching a nuclear first strike.
The only way to avoid this worst-case scenario is for NATO to eschew its invasion plans under any circumstances, including a potential Russian military breakthrough. If they still go through with them, however, then they should keep their forces on the western side of the Dnieper and ideally rely on a neutral mediator like India to convey to Russia that they don’t intend to cross even if they approach it. Anything less is a dangerous game of nuclear chicken that could literally provoke the apocalypse.