Photograph by Jonas Fredwall Karlsson.

If it looks as though Sean Penn is just another Hollywood star courting headlines with a camera-ready cause, look again. With a midlife milestone looming (Penn turns 50 in August), his marriage to Robin Wright Penn seemingly finished (“She is a ghost to me now,” he observes), and a teenage son, Hopper, having recovered from a life-threatening skateboard accident, the Oscar-winning actor decided to redirect his focus and his priorities. Instead of shooting another film or hawking his latest (*Fair Game,*in which he portrays Ambassador Joseph Wilson, playing opposite Naomi Watts as “outed” C.I.A. agent Valerie Plame), Penn ended up committing himself to the people of Pétionville, a once-affluent Haitian suburb where he now runs a camp for 50,000 displaced earthquake survivors.

As *Vanity Fair’*s July issue reveals in detail for the first time, a week after the quake hit last January—killing an estimated quarter of a million people—Penn, a longtime political activist, joined forces with L.A.-based, Sarajevo-born philanthropist Diana Jenkins (creating the humanitarian organization J/P HRO), lined up crisis veteran Alison Thompson to assist in recruiting an A-team of relief volunteers, and flew from his home in Malibu to a ravaged hillside in Port-au-Prince—with a dozen doctors in tow. Ever since, Penn, wearing camouflage khakis and carrying a Glock handgun, has been living in a tent not much larger than an army-surplus locker. And this spring the actor and his organization—who toil alongside Haitian colleagues, fellow aid workers, and army rangers—were designated by their fellow NGOs and U.N. officials as the “camp manager” of the Pétionville facility.Author Douglas Brinkley, the historian, V.F. contributing editor, and a decade-long acquaintance of Penn’s (the pair volunteered in New Orleans in 2005 shortly after the devastation of Hurricane Katrina), traces the humanitarian and personal motivations of the typically press-averse Penn, examining his desire to become an activist in the Phil Ochs mold. “I wanted to give back something more to help struggling people, but I didn’t know how best to do it,” Penn tells Brinkley. “I was for 20 years in a relationship with Robin and 18 years with children. I didn’t have time to commit to anything—for real—in places like Iraq, except to denounce the war. But now I’m single. I can lend a hand.”

For the past five months, Penn’s home base has been the sprawling tent city set up on the former Pétionville country club. (He has left only for a short fund-raising swing, to attend the Oscars in March, and to testify about Haiti on Capitol Hill in May.) As the camp has been buffeted by outbreaks of TB, malaria, dengue fever, and diphtheria, and as the rainy season threatens to bring new potential perils (mud slides, disease, civil unrest), Penn spearheads relief efforts, helps relocate displaced families, and works to arrange deliveries of emergency medical supplies—in one instance coordinating with Venezuelan president Hugo Chávez to airlift in morphine and other drugs.

The U.S. military, it turns out, are among Penn’s strongest supporters. “My politics are not in line with Sean Penn’s,” states Major General Simeon Trombitas, a frequent guest at J/P HRO compound. “But we are allied in trying to save lives and alleviate human suffering. He is a doer and not a talker...and I respect that immensely.” Lieutenant General P. K. Keen, deputy commander of the U.S. Southern Command, concurs: “In a humanitarian crisis you can be a neutral—always pinching your knuckles white. Or you can operate an NGO the way Mr. Penn does.… He intuitively knew how to both work with the U.N. and break its bureaucracy down.... I applaud the leadership he has shown. He doesn’t have to do this.”