It would probably start with Tactical Nukes.
Russia has in writing declared they will never initiate the first use of ANY nuclear weapons, and will only do so in reaction to a foreign nuclear first strike against Russia, and then only if the very existence of Russia is at stake.
America and NATO have not made a similar policy declaration.
Take it for what you will.
Russia has a no first use policy on nukes, so if nukes are used it will be NATO that uses them. Air fuel bombs have made tactical nukes almost unnecessary.