Based on what i learned at university studying the topic of nuclear strategy in multiple defence-related subjects; no, a nuke is a nuke, the line is nuclear weapons of any kind. A nuclear strike on a third (non-nuclear armed) country wouldn’t necessarily trigger a nuclear response in the first place anyway. Mutually Assured Destruction only exists between nuclear armed states.

In my opinion the most likely outcome would be an international military response to repel a mutual existential threat. I’m being purposely vague as we’ve never been down this road so nobody knows what that response would actually look like. But, one possible scenario would be an international coalition force entering Ukraine to directly engage and repel Russian forces, to push them back to their border. There would also be a massive legal dimension to it as well and possibly attempts to apprehend key Russian figures involved on Russian soil.

Maybe we could call it a ‘special military operation’ to keep in the spirit of things?