Washington (CNN)As the lead U.S. nuclear negotiator going toe-to toe-with Iran, Wendy Sherman's colleagues had already labeled her an "iron fist in a velvet glove" and, more pointedly, "badass."
Wendy Sherman negotiated the Iran Deal. Now she's heading to Harvard.
But when an Iranian cartoon portrayed the silver-haired, stern and often intense Sherman as a fox, her staff started calling her the Silver Fox and had T-shirts made emblazoned with the words "Team Silver Fox."
On one particularly rough morning in Vienna, as the negotiations were reaching the end game, Sherman wore the T-shirt to a meeting with her delegation and Secretary of State John Kerry.
It was a badly-needed moment of comic relief for more than a dozen exhausted diplomats who had spent the better part of the last two years sitting at conference tables in European hotel rooms across from Iranian officials trying to hammer out a historic nuclear agreement.
Having reached a deal and with Congress set to vote on it, Sherman announced Thursday she will leave the State Department in October for Harvard University
"This job has been a privilege, but it's been a killer," she said in an interview Wednesday.
In addition to leading the U.S. delegation for the Iran talks, Sherman traveled to more than 50 countries in her day job as undersecretary for political affairs. As the State Department's No. 3 official, she often traveled to places such as Libya, Somalia, Iraq and Afghanistan in between marathon negotiating sessions with the Iranians.
"The last two years were a rollercoaster. The last four years have been the most intense of my life," she said.
Her first career as a social worker and community organizer may seem like odd training for nuclear negotiations. But Sherman said she actually drew upon those experiences with her Iranian counterparts.
Her "caseload" may be more global, but she said the work is similar -- involving the complex relationship and budding detente between Washington and Tehran, as well as managing a series of clients both inside and outside the meeting room.
"That skill set came in handy," she said. "You have to see all the parts in front of you. You really learn how to understand people. "
Sherman had to maintain the unity of the P5+1 negotiating group, no easy feat with Russia and China appearing eager to accept a weaker agreement and France and Britain even more hardline at times than the United States.
Add to that a series of countries not even taking part in the negotiations, but with strong vested interests, including Israel and Persian Gulf states.
Most importantly, the negotiations involved complex and interlocking aspects of a heavily technical nuclear agreement, which is why she would often compare the negotiations to solving a "Rubik's Cube."
"I felt a tremendous responsibility leading this team, but I understood what we were trying to achieve," she said. "There were times I felt overwhelmed and guilty about not being home, but my husband would always remind me how historic it was what we were doing."
Even before she took on the Iran file, Sherman was no stranger to high-stakes nuclear negotiations. As counselor to then-Secretary of State Madeleine Albright, Sherman was at the table when the United States sought a deal with North Korea to curb the development and proliferation of Pyongyang's long-range missiles in exchange for substantial aid and civilian nuclear reactors.
The deal was so far along that Sherman accompanied Albright on a historic trip to Pyongyang, the highest-level visit to North Korea by a U.S. official since the end of the Korean War.
But the White house ended up canceling those negotiations because of the disputed U.S. presidential election of 2000 and the lack of time to follow up with the incoming administration.
In March 2001, Sherman wrote an op-ed in The New York Times urging Clinton's successor, President George W. Bush, to continue negotiations. Instead, Bush cut off direct talks with Pyongyang, later joining a six-nation diplomatic process with North Korea.
Following a 2006 nuclear test by Pyongyang, the so-called Six-Party Talks struck a deal for North Korea to dismantle its nuclear program in exchange for substantial aid and the lifting of U.S. sanctions.
But the long-standing engagement with Pyongyang failed to curb North Korea's nuclear program. In the 13 years since Albright and Sherman visited Pyongyang, North Korea has conducted two nuclear tests and launched countless long-range missiles, and the program continues unabated.
Critics have used the North Korea example as a lightening rod to criticize Sherman and the Obama administration's negotiations with Iran, arguing they were following a failed playbook and would be duped by Tehran, just as the Clinton administration was by Pyongyang.
This time, Sherman was able to bring the high-stakes negotiations to a historic closure.
"I need some time to look back and absorb what we did," she said, adding that the euphoria her team felt when the deal was reached was quickly eclipsed by the enormity of the next herculean task at hand -- selling the agreement to skeptical lawmakers.
At Harvard, Sherman will jointly serve as an Institute of Politics resident fellow and senior fellow at Harvard Kennedy School's Belfer Center for Science and International Affairs
As a close confidante to former Secretary of State Hillary Clinton, her name has surfaced as a possible Secretary of State herself were Clinton to win the White House in 2016. It was Clinton who drafted Sherman back to the State Department from the private sector when she headed it.
Sherman said that although she may advise Clinton in an informal capacity in the future, she has no plans to join the campaign.
"Life comes in seasons," she said. "There will be things I will miss, but I've asked a lot of my family and friends. It's time to take a step back and figure out what I want to do next."
With Sherman's departure, most of the top officials who negotiated with Iran, except for John Kerry, have now left the administration, leaving questions about who will shepherd implementation of the deal if it passes Congress.
Having spent so many hours sitting across from the Iranians, Sherman said that she understands them better but is realistic about the potential for improved relations between the U.S. and Iran.
"It is an opaque society. The level of mistrust is very profound and deep. I think we are taking baby steps, but I don't anticipate great movement right away," she said. "It's not going to happen overnight."