The U.S. does not have any intelligence to suggest such an attack in Ukraine is imminent, the people said. In fact, many DoD officials believe the fighting will stall during the winter months, with neither side able to take much ground.

But in the case of continued battlefield losses, or a complete collapse of the Russian army, some top officials working on the issue have determined that Moscow might resort to employing chemical weapons — including those the country has been associated with using in the poisoning of Alexei Navalny.