Immediately after Russia invaded Ukraine, the odds of Moscow using nuclear weapons were low because it seemed likely they'd overrun the country with conventional weaponry. New York Times national security correspondent David Sanger credits NATO.

"Without the NATO support, I don't think the Ukrainians would have held on," Sanger tells Ian Bremmer on GZERO World.

But now, he warns, we're dealing with the 'Ukraine Paradox': the more successful Ukraine gets, the more likely Vladimir Putin will consider using non-conventional weapons. Will that include nukes? Perhaps.