Signs to watch for if US believes a Russian nuclear attack on Ukraine is imminent

Reflecting the growing concern that Russian President Vladimir Putin may order a limited nuclear attack on Ukraine, the United States and its allies are escalating an already significant standing effort to monitor Russia's nuclear forces.

At least three Navy attack submarines are operating in the Atlantic Ocean area, their primary mission being to track and sink Russian ballistic missile submarines. Another U.S. attack submarine is berthed at a British naval base in Scotland. U.S. and British signal intelligence aircraft are expanding their Ukraine collection efforts to include Belarus and Kaliningrad. Other more boutique U.S....