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Named for the rich Donets Basin of coal seams just below the surface, the region is dotted with Soviet-era mining and factory towns across the sprawl of sunflower fields and grassy plains.

When Russia annexed Crimea and meddled in Ukraine’s Donets Basin, or Donbas, region in 2014, its military revealed new technology, organization and tactics—and upended much of the U.S. military’s thinking about modern warfare.

“We are ready at the moment to exclude our gas supplies to Donbass from our discussions,” he said, referring to the Donets Basin, the contested industrial and coal-mining district of the east.

Q. And how many Russian volunteers were killed in the Donets Basin, including the Urals?

Without the main airport of the Donets Basin, Ukraine’s industrial heartland, under full government control, even the basic practicality of fulfilling his pledge to visit was in question.