What Mr. Tskitishvili did not know was that Ukraine’s military was also arriving at Azovstal. To the Ukrainian soldiers, the plant was a stronghold, surrounded on three sides by water, ringed by high walls, as seemingly impregnable as a medieval keep. It was the perfect place to make a last stand.

“The military never told us, and we never supposed that they would deploy with us,” Mr. Tskitishvili said in an interview. “We planned only for the civilian population, and only as refuge from attack. We did not consider ourselves to be participants in the war.”

For the next 80 days, Azovstal would be a fulcrum of the war, as Russian brutality collided with Ukrainian resistance. What began as an accident — civilians and soldiers barricaded together inside an industrial complex nearly twice as large as Midtown Manhattan — became a bloody siege as roughly 3,000 Ukrainian fighters kept a vastly larger Russian force bogged down in a quagmire that brought misery and death on both sides.

Mariupol stood in the way of one of Mr. Putin’s key aims: the creation of a land bridge linking Russian territory to Crimea, the strategic peninsula in southern Ukraine that Russia annexed in 2014. But the fight also fit the Kremlin’s war narrative. Though several military groups were at Azovstal, many of its defenders were members of the Azov Regiment, a strongly nationalistic group of fighters whose fame in Ukraine and early connections to far-right political figures have been used by the Kremlin to falsely depict the entire country as fascist.