Marine Capt. Ike Fenton during a Korean War battle in 1950. (David Douglas Duncan/Harry Ransom Center)

To photograph the U.S. assault on Okinawa, a World War II battle so fierce it was remembered as a “typhoon of steel,” David Douglas Duncan lay suspended under the wing of a P-38 fighter plane.

Mr. Duncan, a combat photographer with the Marines, was sealed inside a cramped, acrylic-tipped tank designed to transport wounded troops. His camera in one hand, he kept a towel in the other to wipe sweat and condensation from the glass, allowing him to capture the precise moment at which Marine bombers dropped napalm on Japanese pillboxes.

The tank was unventilated, and Mr. Duncan later said the heat was so great he “lost about 11 pounds in 45 minutes.”

Mr. Duncan, who died June 7 at 102, was widely considered one of the finest photojournalists of the 20th century. In Life magazine photo essays, television specials and about two dozen books, he captured the seemingly incongruous subjects of war and art, traveling from the front lines of battle to the treasure troves of the Kremlin in Moscow and the French studio of Pablo Picasso.

A self-described “photo nomad,” Mr. Duncan played a key role in shaping public perception of World War II and the subsequent conflicts in Korea and Vietnam. Many of his photos have been exhibited by institutions including the Whitney Museum of American Art and Museum of Modern Art, both in New York. “He’s really one of the giants of the medium,” said Michael Carlebach, a photographer and photojournalism scholar.

His work in Korea — published in Life, featured in his 1951 book “This Is War!” and later adapted for a set of 22-cent postage stamps — was described by the photographer and museum curator Edward Steichen as “the highest tide that combat photography has achieved.”

Embedded with the Marines in Korea in 1950, he photographed the “thousand-yard stare” of servicemen defending a hill near the Nakdong River, the destruction of Seoul as United Nations forces retook the city and the American retreat from Chosin Reservoir, where temperatures fell to 40-below zero. The weather was so cold, he said, that some of his film “just snapped, like a pretzel.”

Mr. Duncan shot in black and white, with lightweight Leica cameras and Nikkor lenses, made by the Japanese company Nikon, that he helped popularize in the West. He focused on the eyes and inner anguish of such Marines as Capt. Ike Fenton, whose men ran out of ammunition during one engagement with the enemy, and Cpl. Leonard Hayworth, a machine-gunner reduced to tears.

“This Is War!,” Mr. Duncan’s first book of photos, was dedicated in part to Hayworth, who was killed in action one day after seeing his portrait in Life.

“I felt no sense of mission as a combat photographer,” Mr. Duncan told the New York Times in 2003. “I just felt maybe the guys out there deserved being photographed just the way they are, whether they are running scared, or showing courage, or diving into a hole, or talking and laughing.”

His one rule, he said, was to never photograph the faces of the dead, out of respect for their families at home.

Mr. Duncan largely allowed his photographs of the Korean War to speak for themselves, refraining from commentary on the events they depicted. His outlook changed in Vietnam, when he photographed the 1968 defense of Khe Sanh, a Marine outpost that was pummeled for 77 days by North Vietnamese rockets and mortars.

“We seem determined to impose our will and way of life upon most of the rest of the world, whether or not they want it, appreciate it or ask for it,” he wrote in “I Protest!,” a scathing 1968 book that collected some of his Khe Sanh images.

Published by the New American Library for $1, the paperback volume sold about 250,000 copies and placed Mr. Duncan at the fore of photojournalism, alongside Associated Press photographers Eddie Adams and Nick Ut, whose respective photographs of a Viet Cong prisoner’s execution and a naked girl running from a napalm bombing helped turn public opinion against the war.

Mr. Duncan said he took a more artistic approach to some of his Vietnam images — one photo of a wounded Marine illuminated by candles and a lantern recalled the work of Rembrandt — after developing a friendship with Picasso.

From 1956 until the artist’s death in 1973, Mr. Duncan took an estimated 50,000 photographs of Picasso and his work, beginning with an image of Picasso in his bathtub, smiling and scrubbing behind his ear.

Mr. Duncan told the Sunday Times of London that Picasso’s lover, Jacqueline Roque, had greeted at him at the door at that first meeting.

“Without a word she took me by the hand,” he said. “We went past a goat called Esmeralda on the stairs, through a sitting room with a couple of sketches on the wall, through a dark corridor and there was Picasso, just sitting there in a bathtub.”

David Douglas Duncan was born in Kansas City, Mo., on Jan. 23, 1916. His father was a businessman who opened one of the region’s first movie theaters.

Mr. Duncan acquired his first camera at 18 — a 39-cent gift from his sister — and was said to have taken his first newsworthy photo while studying at the University of Arizona in Tucson.

According to some accounts, Mr. Duncan had traveled downtown to photograph a hotel fire and snapped a picture of a man dragging a suitcase out of the smoldering building. The man, he later learned, was gangster John Dillinger — apparently attempting to salvage a stash of stolen money. Mr. Duncan submitted the image to a local paper, which misplaced it.

Mr. Duncan later transferred to the University of Miami, graduating in 1938 with a bachelor’s degree in Spanish and zoology. He contributed to National Geographic, including images of Caribbean sea turtles and swordfish off the coast of Chile and Peru, before joining the Marines in 1943.

While stationed in the Solomon Islands, he met a young Navy lieutenant, Richard M. Nixon. The two reconnected in 1968, when Mr. Duncan photographed Nixon — alone, before a pile of legal pads — crafting his acceptance speech for the Republican presidential nomination.

Mr. Duncan talked his way aboard the USS Missouri at the Japanese surrender in 1945, finding an elevated position to photograph what he later called “a landscape of tranquility.” In short time, a fellow photographer aboard the ship, Life magazine’s J.R. Eyerman , helped him obtain a staff position at Life. Mr. Duncan conducted his job interview while still in uniform.

His military decorations included the Legion of Merit, the Distinguished Flying Cross and the Purple Heart.

Based in Europe and the Middle East, Mr. Duncan covered stories including the 1946 bombing of Jerusalem’s King David Hotel by militant Zionists and the Vietnamese war of independence, in what was then French Indochina.

A 1953 photo essay on that conflict drew the consternation of Henry Luce, Life’s politically conservative owner. Mr. Duncan’s photos and accompanying captions seemed to say, correctly, that the war had already been lost and the days of French control were numbered — a conclusion Luce reportedly found unsatisfying.

Near the end of a contentious two-hour meeting with the publisher, Mr. Duncan told him, “If you don’t like it, then go ahead and fire me.” Mr. Duncan remained on the job but left the publication three years later for Collier’s magazine, dissatisfied with Life’s presentation of a photo essay he prepared on Afghanistan.

A marriage to Leila Hanki ended in divorce. Mr. Duncan married Sheila Macauley in 1962.

He died of a pulmonary infection at a hospital in Grasse, France, according to French news reports. A complete list of survivors could not be confirmed.

Beginning in the 1960s, Mr. Duncan focused on book-length collections of his photographs. Among his most acclaimed works was “The Kremlin” (1960), which featured color photos of Russian artwork and other holdings that were inaccessible to most foreigners. To gain access to the artwork, Mr. Duncan had obtained permission directly from Soviet leader Nikita Khrushchev.

In a review for the Times, Pulitzer Prize-winning reporter Harrison Salisbury, a former Moscow correspondent, described Mr. Duncan as “a man with the camera of an artist, the pen of a poet and a genius for the impossible” and praised “The Kremlin” as “the most beautiful book ever published” about Russia.

Mr. Duncan followed the volume with books about the 1968 political conventions (“Self-Portrait: U.S.A.”), Islamic societies in the Middle East (“The World of Allah”) and the reclusive photographer Henri Cartier-Bresson (“Faceless”) — as well as a slew of books on Picasso, eight in all.

Using a custom-built, silent-shutter camera to avoid bothering the artist, Mr. Duncan captured Picasso painting, dancing in his underwear, jumping rope with his two young children and playing with Mr. Duncan’s pet dachshund, Lump. The dog ended up living with Picasso for six years and was featured in 15 of his “Las Meninas” paintings, reworkings of a piece by Spanish artist Diego Velázquez.

Picasso, Mr. Duncan wrote in a letter to one of his editors, “appears to love all work that he touches. This love shows up in the pictures, too. I have covered many, many subjects as a photographer. This is the Best.”