For more than a decade, it was the deal mining billionaire Ivan Glasenberg had sought more than any other: putting Glencore Plc — the sprawling commodity trader-cum-miner he built — together with industry titan Rio Tinto Group.

For much of the past month, it looked like Glasenberg was finally going to get his way. The two companies were locked in merger talks for at least the fourth time, and nearly everyone involved agreed that they had never been more serious. And then suddenly, with a deadline for Rio to make a firm bid looming, it all fell apart in less than 24 hours.