If one battle more than any other has defined the brutality of Russia’s war in Ukraine, it is the three-month siege of Mariupol’s steelworks this spring, and the harrowing experience of its last defenders. Holed up in Azovstal, one of Europe’s largest metal-producing plants, hundreds of outnumbered, outgunned, wounded and emaciated Ukrainian soldiers, and more than 1,000 civilians, resisted one of Moscow’s fiercest military attacks for more than 80 days. “No one came out of there unchanged,” says Oksana, an Azovstal employee who asked not to give her full name. “They were one person when they went in, and another...