Vladimir Putin has again threatened to use nuclear weapons to protect Russia and its people. In his remarks Friday, he extended that nuclear deterrence over the 15% of Ukraine’s territory that Russia annexed after sham referendums that ended on Wednesday. That even includes some lands already liberated by the Ukrainian army. Mr. Putin’s threats constitute an offensive use of nuclear weapons, imperial expansion through nuclear threats. He must be stopped, but how?

It is tempting for the U.S. to take a leading role. These are not the ramblings of a weakened dictator; Mr. Putin has an idea of how to use nuclear weapons to achieve his purpose. The response from the Biden administration to these serious threats has been swift. National security adviser Jake Sullivan said on Sunday the U.S. would “respond decisively” to a Russian nuclear attack on Ukraine, with “catastrophic consequences for Russia.”