Russian troops mass on Ukraine's border. West worries this isn't like the last time.

It is by now a familiar pattern: Russian troops mass near the Ukrainian border, triggering ratcheting rhetoric and jitters across Europe and the United States that war might be imminent, before a flurry of diplomacy calms nerves.

Officials and experts say that this time is different — and there are several clues giving them genuine concern that the current gambit by Russian President Vladimir Putin might be for real, and the start of a conflict that could drag in Europe and even the U.S. itself.

Putin has deployed more than 90,000 troops along Russia’s...