The prospect of a large-scale nuclear exercise, the presence of sophisticated air defenses in Belarus and elsewhere, and an array of powerful naval assets spread throughout the Black and Mediterranean seas have underscored to Western capitals just how difficult and dangerous any attempted intervention would be.

The Kremlin, said Samuel Charap, a Russia specialist and senior political scientist at the Rand Corp., is looking to “abundantly disincentivize” the alliance even from contemplating coming to Ukraine’s aid militarily. “The way the Russians have thought about this kind of an operation is they have two problems to solve,” he said. “One is the immediate issue of outgunning smaller adversaries along their periphery like Ukraine, and the other is deterring NATO — the U.S., really.”

Russian state news media have reported that the country plans to hold its annual strategic nuclear exercises during the first half of this year, earlier than usual. U.S. and European officials have said they expected those drills to begin this month, potentially to coincide with an invasion of Ukraine, but thus far there are no public indications that has happened.

A senior Western intelligence official, who like others spoke on the condition of anonymity to discuss a sensitive security matter, said that the intent of such an exercise would be to “send a message to the West — that ‘we have strategic capabilities, and if we’re pushed too far, we might use them.’”