Robert watched as his sweat dripped on to the asphalt. Sixty-one, 62, 63. The push-ups and the humidity – it was the first day of a summer heat wave – were starting to get to him. Should he collapse – or just whine? But then all 36 of his fellow trainees would have to start over again. “Damn, this is tough,” Robert thought. Trying hard to keep his rhythm, Robert glanced over his shoulders, at Amber to his right and Alastair to his left. Sixty-six, 67. Amber’s hands and knees were starting to bleed.

They’d been challenged: Quit now, if you’re going to quit. That’s what their instructor, former Navy SEAL John “Jack” Walston, had told them right off the bat. And, sure enough, before the day had even dawned, four trainees had thrown up and dropped out. “Nothing like starting the day with a quitter,” Walston sneered as they fell out.