Few people have lived as long, as varied and as complete a life as David Douglas Duncan. And certainly no photographers ever enjoyed a longer, more varied or more complete career than the Missouri native who became one of the indispensable photojournalists of the 20th century.

Born in Kansas City, Mo., on Jan. 23, 1916, Duncan started taking pictures for newspapers in the 1930s; joined the U.S. Marines after Pearl Harbor; made some of the most indelible photos to come out of World War II and, 20 years later, Vietnam. He documented civil strife and wars in Europe, Africa, Asia and the Middle East, and he captured extraordinary beauty in environments as disparate as the west of Ireland and the deserts of America’s Southwest. He befriended and photographed people such as Picasso and Cartier-Bresson, and he produced the single greatest portfolio of pictures to emerge from the Korean War.

Here, LIFE.com presents a gallery of his celebrated pictures from America’s “Forgotten War.”

Several years before his death in 2018 at age 102, LIFE.com spoke with Duncan about his memories of the conflict — and his hope, in the end, that he might “show something of what a man endures when his country decides to go to war.”

Of his photograph of a Marine captain near the Naktong River after an attack by North Korean troops as ammunition ran dangerously low and reinforcements were nowhere in sight Duncan had written in his 1951 book, This Is War!: “Ike Fenton, drenched and with the rain running in little droplets from his bearded chin, got the news. His tattered Baker Company Marines had only those few rounds in their belts remaining. If the Reds were to launch one more attack they would have to be stopped with bayonets and rifle butts.”

But that attack never came. The Marines held the muddy, blasted, blood-drenched hill. “Radio communication was knocked out by the rain that day,” Duncan told LIFE.com, the recollection still vivid more than six decades later, “and Fenton had to shout most of his orders and sent runners when shouting wouldn’t do. God, he was a cool one. He never lost his head.”

Pictures bring back  visceral memories, and Duncan recounted the circumstances around the making of some of his classic photos in a voice at once firm and touched with wonder at the immediacy of his own recall; at the horrors and heroism he witnessed; at the fact that he was there, recording it all.

“I cabled LIFE’s editors in August [1950] from Tokyo,” Duncan said, “and I told them I was heading back to Korea to try and get what I called ‘a wordless story’ that conveyed the message, simply, ‘This is war.’ Not long after that I was covering the fighting near the Naktong River, and I made the picture of Marines running past a dead enemy soldier, their fatigues absolutely soaked to the chest with mud and muck and god knows what else. And this ended up as the cover image for the book, This Is War!, when it came out a year later.”

From the hellish heat of summer to the arctic freeze of winter, Duncan traveled with Marines, documenting the grinding, torturous lives they led and that troops everywhere have always led in war zones the world over.

“It was forty below zero during the retreat from Chosin Reservoir,” Duncan recalls of one especially appalling battle in the winter of 1950. “And the wind chill! The wind was barreling down from Manchuria and must have made it closer to fifty or sixty degrees below zero. It was so damn cold that my film was so brittle it just snapped, like a pretzel. But I managed to unload and load the camera under my gear and get some film in there, and I got some usable shots.”

The two-week battle at Chosin, while ending with 30,000 United Nations troops fleeing from at least 60,000 Chinese soldiers that surrounded them, is seen as a decisive battle for one primary reason: it showed that outnumbered allied forces could battle through encircling enemy lines, while at the same time inflicting heavy casualties.

While recalling the unspeakable violence and gnawing deprivation of those years, Duncan makes a point of praising the Americans’ South Korean allies. “The thing that comes to mind right away, right now, when looking at these pictures again,” Duncan said, “is that at no time at no time did any Marine feel he had to look around to see what the South Koreans were doing behind him. The Marines in Korea never feared ‘friendly fire’ or artillery coming from the South Koreans from their allies like they did later in Vietnam, fighting with the South Vietnamese. The Koreans could be trusted.”

The Korean War lasted for roughly three years, from June 25, 1950 when North Korea invaded the South until July 27, 1953, when the United Nations Command, the North Korean People’s Army and the Chinese People’s Volunteers signed an armistice agreement. However, the South Korean president, Syngman Rhee, refused to sign the document meaning that, technically, North and South Korea remained at war.

Five years after the end of World War II, American soldiers were fighting again, the time in Korea. Here Marine Capt. Francis “Ike” Fenton pondered his fate and the fate of his men after being told that his company was nearly out of ammunition, 1950.

David Douglas Duncan

American Marines raced past a dead enemy soldier in Korea, September 1950.

David Douglas Duncan

Wounded when a mine blew up his Jeep, an ambulance driver sobbed by the side of the road after learning that a friend was killed in the blast, Korea 1950.

David Douglas Duncan

“Corporal Leonard Hayworth … shows his utter frustration as he has crawled back from his position only to learn that the ammo is gone. Coda: At the last moment, supplies arrived and the men were able to hold their position.” From David Douglas Duncan’s 1951 book, This Is War!

David Douglas Duncan

A wounded American Marine was carried on stretcher improvised from a machine gun, Korea 1950.

David Douglas Duncan

A column of American Marines marched down a canyon road dubbed “Nightmare Alley” during their retreat from Chosin Reservoir, Korea, 1950.

David Douglas Duncan

Marines retreated from the Chosin Reservoir, Korea, 1950.

David Douglas Duncan

American Marines marched down a canyon road dubbed “Nightmare Alley” during their retreat from Chosin Reservoir, Korea, 1950.

David Douglas Duncan

A weary American Marine hooded against the cold during the grim retreat from Chosin Reservoir, Korea, winter 1950.

David Douglas Duncan

A weary, exhausted Marine huddled against the bitter cold during the retreat from Chosin Reservoir, Korea, winter 1950.

David Douglas Duncan

A dazed, hooded Marine clutched a can of food during his outfit’s retreat from the Chosin Reservoir during the Korean War, December 1950.

David Douglas Duncan

The fight for Seoul, Korea, 1950.

David Douglas Duncan

“This,” Duncan told LIFE.com of a picture made during the fight for Seoul, “is the best picture I made in Korea of civilians—a family running down stairs, a father holding a baby, tanks firing away. Those tanks are taking fire from North Koreans right down the street!”

David Douglas Duncan

An American Marine slept in his halted jeep while a puppy whined in his ear during the retreat from the Chosin Reservoir, December 1950.

David Douglas Duncan

American Marines passed bodies of fallen comrades during the retreat from the Chosin Reservoir, December 1950.

David Douglas Duncan

Marines rested after making it through the canyon road known as Nightmare Alley during the retreat from the Chosin Reservoir, December 1950.

David Douglas Duncan

Marines filed past a truck loaded with dead troops during the retreat from the Chosin Reservoir, December 1950.

David Douglas Duncan

LIFE photographer David Douglas Duncan in Korea.

David Douglas Duncan