Ukraine already executed a successful strike on Russian nuclear capabilities. In 2025 Operation “Spider’s Web” completely destroyed several irreplaceable nuclear-capable strategic bombers.
https://www.csis.org/analysis/how-ukraines-spider-web-operat...
There is no "red line".
that didn't cross the red line, and the US has a phone call to talk it over and that Russia's policy for using nukes hadnt been breached