The Russia-Ukraine crisis has been a stark reminder to the world that nuclear weapons could be used in war. Early in the conflict, President Putin put Russia's nuclear forces on high alert and warned the West not to underestimate the risk of a nuclear conflict over Ukraine. How much of a threat is nuclear proliferation, and how can we mitigate the risk of nuclear war? Former US Under Secretary of State for Arms Control and International Security Rose Gottemoeller offers her expert insights.