Nuclear weapons work as a pretty good deterrent because it's literally impossible to know if your opponent will push the button. Even Russians, famous for their disregard for human life, would not take the risk.
> ground invasion that has used only conventional weapons thus far?
And chemical.
And has violated most other rules of warfare (as does every other war). Rape, genocide and murder of civilians is a bit too common in this conflict.
Why? What do you think would be different now?
Do you think Ukraine would have used them against Russia in response to a ground invasion that has used only conventional weapons thus far?