Nothing? Is that the answer? Because nothing is what Russia has done in the face of every single US/NATO "escalation" so far.
What you are so subtly trying to say is that Russia would throw nukes if US troops went into Ukraine. That somehow that is the final "red line" for them. And that's horseshit. The only point where Russia might genuinely consider nukes is if US/NATO go into Russian soil, and that is NEVER going to happen.
If things get to the point where the West sends forces into Ukraine to actively assist in the defence, then Russia will do precisely sod all. Hell, they might even pull back to avoid the chance of getting into an active fire-fight with them. Because they know they are wildly outmatched in any conflict, and they would be starting something that would finish very quickly and very bloodily for their troops.
It's still unlikely that will happen. Because the US doing it unilaterally looks bad, and NATO (as many people have previously explained) is a defensive pact. But stop pretending that ANYTHING happening in Ukraine will end up with Russia throwing nukes. There are no scenarios where that happens.
There are plenty of scenarios where they might threaten it. But that's a different matter.