Jens Stoltenberg, the NATO secretary general, said in response to Putin’s threats: “The answer is not to resign and to stop supporting Ukraine. The answer, if anything, is to further strengthen and support Ukraine. “We will make sure there is no misunderstanding in Moscow about the seriousness of using nuclear weapons. “We have been so clear in our communications with Russia about the unprecedented consequences, about the fact that nuclear war cannot be won by Russia.” He said the only way to end the war is to “prove that President Putin will not win on the battlefield.” Putin’s escalation followed mounting losses and setbacks for Russian forces as they were pushed back by a Ukrainian counteroffensive this month. After the announcement of his mobilization, young Russians started trying to leave the country. Air tickets to visa-free countries such as Turkey and Belarus sold out quickly. There were demonstrations in dozens of cities across Russia, some of which turned violent. In Moscow, protesters chanted: “Send Putin to the trenches.” In the city of Novosibirsk, a young Russian man was filmed shouting: “I don’t want to die for Putin.” As of Wednesday night, 700 people had been arrested by Russian authorities in more than 35 cities.