Russia has a functional ABM system. They have S400 systems all around the country at important installations and cities. They are being replaced with S500 systems. These systems are interconnected.

It doesn't matter if they can actually win a nuclear war or not. If they believe they can, they might start one.

They are already using nukes in a war today. By threatening to use their nukes, they are making use of them.

Perhaps the definition of nuclear war should be expanded to include such uses.