Catherine Leroy was 21 when she arrived in Vietnam in 1966 with only a hundred dollars, a Leica M2 and a limited professional portfolio. Over the next three years covering the war, she built an exceptional body of work: surviving and documenting a capture by the North Vietnamese Army, parachuting in combat operations with the 173rd Airborne, and being published on the covers of major magazines, including Life and Paris Match.

She was also the only female photojournalist then working in Vietnam, after the death of Dickey Chapelle, who was killed by a grenade in 1965.

Ms. Leroy wrote over 100 letters home during her three years in Vietnam, one almost every 10 days, and they show how she was processing her experiences and navigating professional challenges. They are candid and revealing, attesting to her resiliency, exposing insecurities, and showing her in moments of triumph, despair, optimism or courage.

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Despite her many strong images, Ms. Leroy had remained relatively unknown, partly because she wasn’t a self-promoter, and partly because women photographers have often been excluded from the medium’s history, especially war photography. Now a selection of those letters are featured on a new website, accompanied by some of her archive and details from her life. It will be followed by the documentary “Cathy at War” by the writer and filmmaker Jacques Menasche.

Most of the letters are to her mother, Denise, focusing at first on topics that read like holiday postcards – gifts she hoped to send home, people she met, her tan. To her father, Jean, she wrote mostly about business — publications that bought her work, logistics and future plans. She frequently reassures them, noting in one letter to her mother that “if anything happens to me, you’ll be informed within 24 hours. So don’t worry if I sometimes leave you without any news.”

Her tone changed over time. In the early letters, she was unsure whether she should stay or go home, even as she felt less insecure about her technical skills. She spoke of the relationships she was forming with the military and about daily life. On Sept. 4, 1966, a few months after her initial arrival, she wrote to her mother:

I celebrated my 22nd birthday at the Da Nang press office in the company of lots of Marines. I had left Saigon a few days earlier with an AP reporter for an operation near the DMZ. On this occasion he opened a bottle of French champagne when we got back. I only got back last night, Saturday, very tired…I’m getting an iron reputation with the marines. I’m very proud of it. I’m leaving tomorrow, Tuesday, morning for the Delta. Subject: a village a few days from the election. The war seems to be calming down, as always happens just before political events. The Delta is the key region. I’ll stay there two or three days. After that, I don’t know. I’m preparing a few photos of me taken in various locations and will send them to you. I haven’t forgotten your wig, I’ll take care of it too. Don’t be angry with me. I’m always away: back in Saigon for two days, get something cleaned, tidy up, wash, everything is dirty.

As her confidence grew, she wrote about her professional successes. But the letters also showed low moments, like being unable to get the pictures she wanted in the field or leaving an envelope of her best prints in a taxi after fending off a robbery attempt from the driver.

The letters testify to the challenges women encountered in the field. While there were female journalists in Vietnam, Ms. Leroy was the only woman photographer after Chapelle’s death, and faced resistance from the military and male journalists. Robert Pledge, president of the board of the Catherine Leroy Fund, a not-for-profit organization in Paris dedicated to her work, said many prominent photojournalists questioned why a young woman wanted to cover war.

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She wrote about how people were nice when they first thought she was an unintimidating, five-foot-tall, pigtailed girl. But when they realized she was a serious photographer, their opinions changed. “Many were very, very negative and very nasty with her,” said Mr. Pledge, who is also co-founder and director of Contact Press Images.

Writing to her father on May 13, 1967, after noting her successes in being published in leading magazines, she marveled how “I’m getting telegrams from people who I didn’t know existed for years, while at the same time I’m hated more than ever by known and unknown enemies in Saigon, American civilians, as well as servicemen in Saigon.”

One photographer told Mr. Pledge that the male photographers “were really nasty chauvinistic pigs.” That same photographer, who now is a filmmaker, called her late in life to apologize.

Still, she found the opportunities available to her in Vietnam surpassed anything back home in France. Like parachuting with the 173rd Airborne Brigade — the 85th jump she had taken in her life, after receiving her parachuting license as a teenager.

“I am the first woman to jump in Vietnam during this first operational jump by American parachutists since the war began,” she wrote to her father on Feb. 23, 1967. “I’m very proud to have jumped with the Americans here, it’s a big professional success in every way. I now know that I will be able to work in the United Sates one day without too much problem.” She wrote how she planned to submit her work to major competitions. “I’ve always thought I should succeed because I never gave in,” she continued.

Many consider Ms. Leroy as the most daring of all photographers in Vietnam, and she most likely spent the most time in combat because she needed the money. Being broke meant traveling with soldiers, sharing rations and sleeping in the countryside.

As Mr. Pledge noted, “If Robert Capa was saying that it’s not good enough because it’s not close enough — she was very close.”

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