Pham Thanh Tam holds up his old army helmet, a testimony to his war years. It is rusty and riddled with holes. "I know you think these are bullet holes," he says. "If they were bullet holes I would be dead. I made these holes after the war. I used the helmet as a watering can for the garden." This 67-year-old war artist and colonel in the North Vietnamese army is a veteran of the Indochina (1946-54) and Vietnam (1965-75) wars. His energy and zest for life got him through 30 years at the front and some of the deadliest battles of both wars. Today his special brew of fermented banana juice gives him a little extra boost.
"These legs have hiked thousands of miles up and down the Ho Chi Minh Trail, all the way to Halong Bay, and down to the south during the Tet offensive. I travelled everywhere to sketch the war," he says.
His first sketches were of the battle of Dien Bien Phu (1954), where he fought against the French in the trenches for 55 days and nights. Under fire, he developed a talent for telling a story with a few brushstrokes in a matter of seconds: a picture of a medic looking after a wounded soldier communicates compassion and humanity in war. He learned the croquis rapide (quick sketching technique) in the Viet Bac, Ho Chi Minh's remote jungle headquarters during the Indochina war. His teachers were some of the most talented artists of their generation. Inheritors of the French neo-classical style introduced to Vietnam in 1925, they followed Ho Chi Minh into the jungle and set up an art school there to teach "art for the people", a mix of the French style and Soviet-inspired socialist realism. Some of the results can be seen in the exhibition Vietnam Behind the Lines, which opens at the British Museum this week.
After attending the Hanoi Institute of Fine Arts in the inter-war years, Pham Thanh Tam was commissioned by the army to be a war artist during the Vietnam war. Wherever he went he carried his sketchbook in his backpack. The quick sketching technique he developed at Dien Bien Phu helped him survive one of the fiercest battles of the Vietnam war, at Khe Sanh in 1968, where he drew hundreds of drawings and watercolours. For two months he lived underground, suffering constant bombing, napalm and artillery fire - which killed 10,000 North Vietnamese soldiers and Vietcong units.
"At Khe Sanh I did the watercolours right there at the battle. I used a plume rehaussée d'aquarelle [pen with watercolour]. The idea was not to hang around too long in one place. So I would usually add the pen later on. When I travelled around the country and there was no fierce fighting, I had more time to sketch carefully. I travelled with a photographer. That was very useful. If I forgot details, I could look at his pictures and fill in details. I used all kinds of materials: watercolours, pens, pencils - whatever I could find."
Pham Thanh Tam managed to preserve most of his drawings by sending them back to Hanoi for safekeeping. Many artists did not. Nguyen Toan Thi, now the director of the Ho Chi Minh City Fine Arts Museum, lost most of his drawings at the battle of Am Giang in the south of Vietnam, near the Cambodian border. He remembers the date: February 1971.
"I carried what I owned on my back because we moved camp every two to three days. A hammock, a mosquito net, an extra set of clothes, my rice belt with 15 days worth of provisions, my ration of Chinese military biscuits in case we couldn't cook the rice (because the enemy would spot the smoke), a hand gun and a long gun, ammunition and, of course, my folder with all my drawings and my pens and pencils."
The battle was fierce, so he hid the heavy folder with his drawings to lighten his load. Several of his friends were killed or captured. He stayed concealed until he thought the enemy had left. Then a voice boomed out of a helicopter gunship hovering right above him: "We have captured two kilos of information from a VC artist. Kill him so he cannot draw any more." He ran for his life. His drawings were never recovered.
Given the harsh conditions at the front, it is extraordinary that war artists managed to produce any work at all. Hunger, malaria and living underground were commonplace. Nguyen Van Tru is a quiet and reflective artist, and expert in Chinese brush technique, which he learned in Shanghai after he graduated from the Hanoi Institute of Fine Arts. He was also head of the Vietcong arts propaganda unit in Long An, 30km from Saigon, a landscape of water canals and dense tropical jungle. He lived clandestinely underground from 1968 to 1975. His images of puppet shows, card games and music lessons appear oddly idyllic. For him, drawing was an escape from a "cruel and gruesome" reality.
"I followed the soldiers into battle. I carried a small gun and a long gun to protect myself. I covered myself in plastic to cross the river. Life was very hard. Your clothes were wet all the time. I carried the drawings with me. During the battle I just drew quick lines and sketches. I would then fill them in back at base camp. If you lived in Long An, you lived under a village house in a hole. We were three or four down there. At first the owners of the house used to just cover the trap-door to our hiding place with grass. But once the adversary found out about the underground hiding places, they had to cover the opening with a lot of dirt. That was dangerous. A friend died that way. Another friend lost his arm when he stuck his hand out of the trap door holding a live grenade.
"I couldn't draw down there. It was dark. There were no tables or chairs. I drew in my free time, usually at night, when we could come out of our hiding place. I used a Chinese ink brush and watercolours. At night it was so peaceful; I would draw then."
War-related subjects, popular in North Vietnamese art, cover every aspect of life, both behind the lines and at the front. Drawings that depict activities behind the lines show life continuing as normal: rice cultivation, the manufacture of enamelware, newspaper printing, puppet shows to entertain the troops, and soldiers hanging out in their hammocks or getting their hair cut. At the front, common scenes included the transport of equipment and ammunition, militia women manning anti-aircraft defence guns against American B-52s, guerrilla warfare in the south of the country and medics caring for the wounded, as well as war-torn landscapes after bombing and napalm attacks. Portraits of regular soldiers, militia men and women, youth brigade volunteers, doctors, nurses, engineers and communication experts were in great demand and show the extent to which this was a "people's war".
Portraits were also popular, as they provided their subjects with a way to be remembered by loved ones if the worst should happen. Quach Phong, who operated mainly in the mountainous regions of Vietnam, remembers that "soldiers liked having their portraits done. It made them feel part of history."
In Hanoi artists had access to larger pieces of paper and better utensils. Tran Trung Tin, who was not an official artist, managed to find oil paints and used newspaper for his haunting cityscapes of Hanoi during the Christmas bombings of 1972.
Thanh Ngoc, a wonderful portraitist, was based in Hanoi during most of the war. Her portraits and watercolours of militia women at the controls of anti-aircraft guns, or of a Hanoi street with women inspecting bomb shelters, reflect her more sedentary life.
The war artists' drawings rarely reveal the terrible conditions in which they lived, or the horrific injuries they witnessed while working under fire - limbs torn off, bodies shattered and burned by blockbuster and phosphoric bomb attacks. Van Da, a colonel in the North Vietnamese army and a graduate of the Hanoi Institute of Fine Arts, depicted dramatic scenes of destruction after the bombings, using charcoal or Chinese black ink pen without any graphic narrative details. His feeling was that every Vietnamese had someone who was close to them who had died. It was always somebody's father, son, brother, mother or sister. And that was just too personal a tragedy to draw.
"Bombs exploded people's stomachs," he recalls. "I just didn't have the nerve to draw that. Every Vietnamese family has someone who died during the war. In general, there should be no war."
· Sherry Buchanan is the author of Vietnam: Drawing Under Fire, published by Asia Ink this autumn at £25. Vietnam Behind the Lines: Images from the War 1965-1975 is at the British Museum, London WC1 (020-7323 8000), from Thursday until December 1.