What happens if Russia nukes Ukraine?

How should the US respond if Russia uses a tactical nuclear weapon in Ukraine?

Unlike strategic ones, tactical nukes are not subject by signed treaties, so all bets are off, New York Times national security correspondent David Sanger tells Ian Bremmer on GZERO World. Independent agencies don't inspect them so we don't know very much about their size, range, effects, or pre-launch prep.

As for the 'Mutually Assured Destruction' dynamic that held nuclear war at bay for 60 years, including during the Cuban Missile Crisis? Sanger says the dynamics now are "completely different."

Since Ukraine is not a member of NATO, it's not clear if a Russian nuclear attack there would trigger a major US response.

Watch the GZERO World episode: US threat levels from foreign and domestic enemies