New Delhi, Aug. 1 -- KYIV - A Ukrainian soldier serving in Kostiantynivka put it plainly this week. "Things are very bad in Kostiantynivka," he told reporters. "We've effectively already lost it."

His assessment, delivered as Russian forces extended their presence across most of the city that Ukraine had designated one of four defensive fortresses in Donetsk region, arrived in the same week Ukraine fired both its defense minister and its military commander-in-chief. The coincidence was not accidental. The loss of the fortress and the loss of the command structure reflect the same underlying pressures.

The Institute for the Study of War estimated that Russian infiltration of Kostiantynivka had spread to most of the city by Thursday, up f...