Still, using a nuclear weapon could very likely undermine Mr. Putin at home.
Even before the war, U.S. officials began warning about various possible Russian “false flag” operations that Moscow was hoping to use to create a pretext for the invasion. Russia, for example, said Ukraine was planning to use a chemical weapon, and then American intelligence uncovered a plot by Russia to hire crisis actors to create a false pretext for invasion.
Pentagon officials were on edge Monday after three phone calls in four days between the Defense Department’s top civilian official and top uniformed officer. On Friday, Mr. Austin initiated a phone call with Mr. Shoigu, his Russian counterpart, the first time the two men had spoken since May.
The conversation was meant to delineate the red lines that could potentially provoke Russia to launch a nuclear attack on Ukraine and to clarify for the Biden administration why Mr. Putin has been raising the prospect of a nuclear strike in Ukraine, three officials said.
Defense Department officials were surprised when two days later, Mr. Shoigu requested another call, at 7:30 a.m. Sunday, in which he accused Ukraine of preparing to use a dirty bomb, two officials said.
The allegation, which the United States has said was baseless, spooked senior defense and military officials, who expressed concern that Moscow might be using the false flag as a distraction, masking some other more ominous development.
That possibility only heightened concerns among already jittery senior Pentagon officials about Russia’s next possible step up the escalation ladder. One senior U.S. official said there were new, troubling developments involving Russia’s nuclear arsenal. The official asked for anonymity and declined to provide any details, given the sensitivity of the issue.
At a Pentagon briefing on Monday, a senior U.S. military officer said there was no indication that Mr. Putin had made a decision to use unconventional weapons — nuclear, chemical or biological arms — but offered no details.
