Landler’s account shows how early and extensively Clinton and her State Department staff were involved in the Iran talks, despite her initial wariness. And in a campaign in which Donald Trump often advocates a blunderbuss approach to foreign affairs, this story is a reminder that breakthroughs often come via strange and invisible pathways — ones that, in this case, the administration sometimes sought to obscure.

The Ismaily contacts began in May 2009, just four months after President Obama had taken office, when Dennis Ross, a top adviser to then-Secretary Clinton, met the 51-year-old Omani at the State Department.

At that first meeting, the Omani surprised the Americans with “an offer by Iran to negotiate” about the nuclear program, writes Landler. Obama had already sent a secret letter to Iran’s Ayatollah Ali Khamenei proposing negotiations but had received a diffident response. “Ismaily assured Ross he could bring the Iranians to the table” and that Oman would be “an ideal venue for secret negotiations.”