One of the mysteries of Campaign 2016 is why the Iran nuclear deal has vanished as an issue. But a new book reveals some startling details about how the diplomacy with Tehran began in secret, long before reformers took power there, and the crucial role played by presumptive Democratic presidential nominee Hillary Clinton.

The diplomatic narrative is laid out in “Alter Egos,” by New York Times White House correspondent Mark Landler. He’s the first to disclose the full extent of Oman’s “back channel” to Iran that opened in 2009 through a colorful fixer named Salem ben Nasser al-Ismaily.

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 where 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.”

Both promises turned out to be true. First, though, came the uproar of the Iranian presidential election in 2009 and the brutal suppression of the “Green Revolution.” Some critics have argued that Obama’s eagerness for a diplomatic opening to Iran blunted the U.S. response to the stolen victory by hardliner Mahmoud Ahmadinejad.

But the Omani mediation track continued. After Iran seized three American hikers in July 2009, Ismaily secretly began negotiating their release. The first was freed in September 2010, the other two a year later. Ross and a colleague traveled to Oman in December 2010 to hear more about the channel. Clinton had a similar exploratory talk with Oman’s sultan in January 2011, though she wrote that she initially saw the opening as a “long shot.”

John Kerry was jumping into the Oman channel even before he became secretary of state. He got to know Ismaily during the hiker negotiations and made several visits to Oman in 2011 and early 2012. Kerry also met the Omani intermediary in London, Rome and Washington.

“In his zeal to jump-start the negotiations, Kerry passed several messages to the Iranians through Ismaily,” according to Landler. One of these messages may have been crucial: Kerry, still a senator and thus not formally speaking for the administration, suggested that under a nuclear agreement, the Iranians would be able to enrich uranium — Tehran’s baseline demand. “In some ways, Kerry and his enthusiastic Omani go-between were merely cutting to the chase,” writes Landler.

More secret meetings through the Oman channel followed in 2012 with Clinton’s top aides, Deputy Secretary Bill Burns and Deputy Chief of Staff Jake Sullivan. Then, in 2013, the train began to accelerate with Kerry’s appointment as secretary of state and Hassan Rouhani’s election as president of Iran. By the end of that year, an interim nuclear agreement had been reached.

Sullivan explained in an email that although Clinton was skeptical at first about the Omani contacts, they proved important: “Without that channel, we likely would have spent the fall of 2013 trying to figure out who to talk to and how.”

Landler’s book also underlines the question of whether the administration’s media campaign, led by deputy national security adviser Ben Rhodes, emphasized the post-Rouhani chapters in its public telling of the story, and obscured the largely unnoticed early contacts through the Oman channel. A New York Times Magazine profile of Rhodes argued last month that this impression “was largely manufactured for the purpose for selling the deal.”

The administration’s hand was also visible in the State Department’s deletion of footage of a December 2013 press briefing asking about secret negotiations with Tehran.

The Iran nuclear agreement deserves more attention in this campaign. Kerry and Obama may have concluded it, but Clinton helped get it started. Trump needs to explain why the world would be safer without this deal, and how he would have negotiated a better one. And the administration needs to explain why it opted for secrecy on a landmark agreement.

David Ignatius’ email address is davidignatius@washpost.com.