If Putin uses a tactical nuke, it's World War III

Once you see the mushroom cloud, “no one will know whether it was a 20-kilaton weapon or a 1 megaton (1000 times stronger),” said Joshua Pollack, editor of the Nonproliferation Review, published by the James Martin Center for Nonproliferation Studies.

“They’re more similar than you might expect in terms of their destructiveness,” Pollack told The Daily Beast. “Even this small nuke is extremely destructive depending on where you drop it. It would be a very large explosion that would generate an electro-magnetic pulse, and it would probably start fires.”…

“The TACs are new, they weren’t there during the Cold War,” Korb added. “We just had the big ones. In theory, the U.S. would not respond with a big one, but you don’t know that. If Putin launches one with a smaller yield, he doesn’t know if we will respond using a strategic weapon. That’s deterrence. He doesn’t know that, and we want to keep him not knowing that.”