Paul Nash was one of Britain's most important Modernist painters. He was also a war artist, photographer, depressive, mystic, Romantic, and naturalist. His visionary works redefined our idea of landscape painting. Engaging with new European avant-garde movements such as Surrealism and Vorticism, they also worship and mythologise England and its ancient past.

This autumn, his soulful, disarming surrealist landscapes have come to rest at the Tate Britain, bringing with them a marvellous revelation.

Nash was beguiled by the mysticism of Samuel Palmer and William Blake, and the show opens with dark spiritual landscapes that hark to both painters.

His best-known works, though, are the war paintings. In 1917, he became the official artist of the First World War. The muddy canvases, some of the most familiar of the war, are inky with a stygian despair. They show Ypres as a broken land, churned and shell-pocked beyond recognition. Nash was outraged at the loss of life, but also the destruction of landscape. In a letter from November of that year, writing from the front, he wrote:

"Sunset and sunrise are blasphemous, they are mockeries to man... Unspeakable, godless, hopeless. I am no longer an artist interested and curious, I am a messenger who will bring back word from the men who are fighting."

Returning from Ypres, a traumatised Nash retreated into the landscape of Britain: burial mounds, standing stones, legends, ancient woodland. These works are almost pantheistic, exploring the life-force in seemingly inanimate objects such as trees, rocks and bones. These ideas were realised through the juxtaposition of found objects with landscape to create mysterious encounters, in paintings such as Event on the Downs and Equivalents for the Megaliths.

The rooms dedicated to Nash's contemporaries and artistic alliances, including Ben Nicholson, Barbara Hepworth and Henry Moore, are not to Nash's benefit. Next to the vivid colours and strutting sculptures, Nash's sober works seem drained and pallid.

It is in the final room, though, that the exhibition really gets to the heart of this remarkable artist. It is a nocturnal space; the walls are painted a midnight blue, and the paintings that hang on them gaze heavenward. In his twilight years, Nash became obsessed by “the spaces of the skies and miraculous cloudscapes that constantly form, change and disappear" and the "compelling magic of moon and seasons and solstice". We see shooting stars, mystical suns and moonlit landscapes that summon up the hidden magic of human experience. It is very moving.