The U.S. Army soldier steered the drone deep into enemy territory where it spotted about two dozen enemy vehicles hiding beneath a canopy of trees.

Col. Joshua Glonek recalled the jolt of excitement that ran through his staff at the drone’s discovery, followed by hushed chatter in the small, dark tent where his team was preparing for what came next.

His 3,500-soldier brigade was in the last hours of an 11-day training center battle against a similarly sized force. Such exercises — the closest thing the Army has these days to actual combat — happen many times a year.

But this one was different.

The rapid proliferation of deadly drones in places like Ukraine had set off a growing sense of alarm among the Army’s top leaders.

Senior Army officials were relying on Colonel Glonek and his troops to catch up to America’s adversaries. It was their job to figure out which drones the Army should buy and how it should fight with them.