The US must do what it can to prevent Russian military from crossing the nuclear threshold, Scott Sagan argues.

Nuclear weapons are not just a force used to deter another state from attacking, they can also be a shield behind which one can engage in aggression, says Sagan, the co-director of the Center for International Security and Cooperation at Stanford University. Sagan’s research examines nuclear strategy, nuclear proliferation, the ethics of war, and public opinion of combat.

“…Putin is the most dangerous man in the world.”

Russian President Vladimir Putin’s reminder of Russia’s nuclear arsenal at the beginning of its recent invasion into Ukraine is a warning to the United States and other NATO members that if they get directly involved in the conflict, there could be a risk of nuclear escalation, Sagan says.

It is estimated that Russia’s stockpile includes 4,477 nuclear warheads, according to a report by the Bulletin of the Atomic Scientists.

Here, Sagan discusses Putin’s nuclear threat and what could happen if Russia crosses the nuclear threshold, a line that hasn’t been crossed since 1945, when the US detonated two atomic bombs in Japan: