ANDRII TKACHUK KNELT BEHIND A LINE OF BUSHES, an antitank gun perched on his shoulder, awaiting a convoy of Russian troops. It was eerily quiet; there was no wind, no birdsong. Just empty fields. He looked through the weapon's sight and steadied himself. He had never fired this gun before, and couldn't afford to miss.

Tkachuk isn't a trained soldier. He's a runner, and not just any runner: He's one of the world's greatest ultramarathoners. He signed up for active duty after Russia invaded his native Ukraine in late February 2022, and a few days later, on March 1, he marched toward Zaporizhizhia in southeast Ukraine with the 128th Mountain Assault Brigade. While he was familiar with the suffering that can come while running 200 kilometers or more, he was unprepared for the horrors that awaited him.

Almost immediately his unit was decimated by Russian fire. For the next few days, Tkachuk survived by doing what ultrarunners do-he kept moving. He trudged across an empty field in subzero temperatures, escaped Russian helicopters by diving into a barn. At one point a missile slammed into a house in front of him, sending shards of wood as far as 200 feet in the air. "I'd only seen this in war films," he says.

Then five days after he landed in Zaporizhizhia, Tkachuk found himself trapped. He'd ended up in Shcherbaky, a strategic village halfway between the Dnipro River and the Ukrainian stronghold of Orikhiv. When he arrived, other Ukrainian soldiers, many disconnected from their own units, were "in a bad mood," he remembers, and some considered deserting, but that seemed unwise. Russians had been spotted a couple of miles away and were believed to be advancing.