EXPLAINER: How real are Putin's nuclear threats in Ukraine?

Russian President Vladimir Putin warns that he won't hesitate to use nuclear weapons to ward off Ukraine's attempt to reclaim control of Moscow-occupied areas that the Kremlin is about to annex.

While the West dismisses that as a scare tactic, a top Putin lieutenant upped the ante by boldly saying the U.S. and its NATO allies wouldn't dare strike Russia, even if it used nuclear weapons in Ukraine.

A look at the nuclear threats over Ukraine:

WHAT DROVE PUTIN TO MAKE THE THREATS?