Heightened nuclear fears in October came against the backdrop of a successful counteroffensive by the Ukrainian military when it reclaimed a huge swath of territory east of Kharkiv, in the northeast. It then made a drive at Kherson, in the south, forcing the Russian military to eventually retreat from there. With their army in disarray, Mr. Putin and other Russian officials warned against Ukraine’s use of a so-called dirty bomb: a crude device that spreads radiological material but does not create a nuclear reaction. US officials were unsure what Moscow might do.
As winter set in and Russia managed to pull its forces from Kherson in a relatively orderly retreat, the battlefield stabilized. Intense fighting remains around Bakhmut, in the Donbas region, but there are no drastic territorial shifts. In the south, the Russians have dug in, intensifying their defenses; they do not appear to be on the brink of a collapse that could make their leaders think that only the use of a tactical nuclear weapon could stave off defeat.
US officials also credit an improved dialogue with Moscow, at least over nuclear issues.
Amid Russia’s battlefield failures, US intelligence concluded that Russian military officials had discussed situations in which a tactical nuclear weapon could be used, Two calls between Defense Secretary Lloyd J. Austin III and the Russian defense minister, Sergei K. Shoigu, worried Washington because Mr. Shoigu had raised concerns about Ukraine’s possible use of a dirty bomb,
The claims were propaganda, but some US officials said Russian officials appeared to believe their own disinformation. Getting International Atomic Energy Agency inspectors into Ukraine — and, in early November, when the agency found no evidence of a dirty bomb — helped ease tensions.
A call in late October between Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, and his counterpart, Gen. Valery V. Gerasimov, Russia’s chief of the general staff, also relieved tensions. In the call, according to two US officials, General Gerasimov outlined a use of nuclear weapons consistent with Washington’s understanding of Russia’s nuclear doctrine.
Mr. Burns also met with his counterpart, Sergei Naryshkin, the director of Russia’s foreign intelligence service, in Turkey to warn Russia about its nuclear threats. The purpose of the trip, Mr. Burns said on Thursday it was “to make very clear the serious consequences of any use of tactical nuclear weapons.” The meeting, officials said, opened up a new line of communication with the Russian leadership.
President Biden has been criticized for being overly cautious in sending assistance to Ukraine, but US officials insist his top priority is ensuring that the war does not escalate into a nuclear conflict between Russia and the West. And while American officials have a better sense of what actions will prompt Russian reaction, determining what might provoke Mr. Putin is imperfect.