When she left her role as the Undersecretary of Defense for Policy in early 2012 to spend more time with her children, she was the highest-ranking woman there. Despite the gains of women at the Pentagon, the Defense Department remains a stubbornly mostly-male bastion.

In her first stint in the Department of Defense under Bill Clinton, there were so few females that a women’s leadership lunch—which could be accommodated by a small round table—spawned conspiracy theories. A decade later, when she served Obama, there were enough women in the Defense Secretary’s office that they came to be known as “the feared clackers”—a reference the sharp sound of heels on the Pentagon’s echoing floors.

“You know you’ve arrived when someone creates a nickname for you,” she joked in an interview with me at the Fortune Most Powerful Women dinner at the State Department last spring. Her barbed humor should come as no surprise; as a child she spent Fridays on the sets of “I Love Lucy” and “The Odd Couple,” has her cinematographer Dad directed episode.

But Flournoy’s view of the world is dead serious. In our interview at a Fortune Most Powerful Women dinner in Washington last March, she worried that cuts to the defense budget were hurting the nation’s security. “It’s not a terribly peaceful environment,” she said. “We need a robust military as a deterrent.” One of her greatest national security concerns—that unsecured nuclear weapons will fall into the hands of terrorists.

Flournoy has a reputation for fitting her concerns into a broad worldview. The think tank she co-founded, Center for a New American Security, looks at issues like the nation’s growing energy independence in the broader context of America’s evolving place in the world. “In the 21^st century national security has to get framed in broader terms,” she says.

For more on Flournoy’s view of the world, check out our interview: