Several people here have claimed that a Russian use of tactical nuclear weapons would be a "game changer." Okay.
A game changer is an idea or an event, or procedure that effects a significant shift in the current manner of doing or thinking about something.
So what would change?
I gave the example in Ukraine of a series of nuclear strikes to open a gap and destroy their supporting logistics in order to return to offensive operations in eastern Ukraine. Great. The game has changed. What would happen?