Heightened nuclear fears in October got here in opposition to the backdrop of a profitable counteroffensive by the Ukrainian navy when it reclaimed an enormous swath of territory east of Kharkiv, within the northeast. It then made a drive at Kherson, within the south, forcing the Russian navy to finally retreat from there. With their military in disarray, Mr. Putin and different Russian officers warned in opposition to Ukraine’s use of a so-called soiled bomb: a crude machine that spreads radiological materials however doesn’t create a nuclear response. U.S. officers had been not sure what Moscow may do.

As winter set in and Russia managed to tug its forces from Kherson in a comparatively orderly retreat, the battlefield stabilized. Intense combating stays round Bakhmut, within the Donbas area, however there are not any drastic territorial shifts. Within the south, the Russians have dug in, intensifying their defenses; they don’t seem like on the point of a collapse that would make their leaders suppose that solely using a tactical nuclear weapon might stave off defeat.

U.S. officers additionally credit score an improved dialogue with Moscow, a minimum of over nuclear points.

Amid Russia’s battlefield failures, U.S. intelligence concluded that Russian navy officers had mentioned conditions through which a tactical nuclear weapon might be used. Two calls between Protection Secretary Lloyd J. Austin III and the Russian protection minister, Sergei Okay. Shoigu, anxious Washington as a result of Mr. Shoigu had raised issues about Ukraine’s potential use of a grimy bomb.

The claims had been propaganda, however some U.S. officers stated Russian officers appeared to imagine their very own disinformation. Getting Worldwide Atomic Vitality Company inspectors into Ukraine — and, in early November, when the company discovered no proof of a grimy bomb — helped ease tensions.

A name in late October between Gen. Mark A. Milley, the chairman of the Joint Chiefs of Workers, and his counterpart, Gen. Valery V. Gerasimov, Russia’s chief of the overall workers, additionally relieved tensions. Within the name, in accordance with two U.S. officers, Normal Gerasimov outlined a use of nuclear weapons according to Washington’s understanding of Russia’s nuclear doctrine.

Mr. Burns additionally met together with his counterpart, Sergei Naryshkin, the director of Russia’s international intelligence service, in Turkey to warn Russia about its nuclear threats. The aim of the journey, Mr. Burns stated on Thursday, was “to make very clear the intense penalties of any use of tactical nuclear weapons.” The assembly, officers stated, opened up a brand new line of communication with Russian management.

President Biden has been criticized for being overly cautious in sending help to Ukraine, however U.S. officers insist his high precedence is making certain that the struggle doesn’t escalate right into a nuclear battle between Russia and the West. And whereas American officers have a greater sense of what actions will immediate Russian response, figuring out what may provoke Mr. Putin is imperfect.