Russia’s Military Buildup Near Ukraine Is an Open Secret

WASHINGTON—Russia faces a very 21st century challenge as it piles up forces near Ukraine’s border: Much of its military operation is being carried out in plain sight.

Taking on a role once reserved for spies, amateur sleuths and analysts with private and nongovernmental organizations are tracking Russia’s buildup day by day, mining commercial satellite images, social media posts and flight-tracking data to compile a strikingly precise picture of Moscow’s deployments—and of the U.S. military’s efforts to monitor them.