John and Paul Nash had an experience common to many brothers who are close in age and interests in that, at least early in their careers, they were often lumped together as if they were physical manifestations of the same sensibility— like some avant la letter version of Gilbert & George.

Pity the slightly less talented sibling, therefore. This revealing two-room exhibition goes some way towards redressing the balance in John’s favour by presenting the brothers’ landscape paintings (none of their war art) side by side and without favour. Paul’s work is more intense, more experimental and—for most people—ultimately more engaging, which is why his reputation has so eclipsed his brother’s. A work such as the watercolour Tench Pond in a Gale (1921–2) is an intimation of apocalypse, capturing the swirling movement of ash trees in the wind.

But John’s depictions of the English landscape possess their own distinctiveness, a kind of serene plasticity in which landscape elements appear to be moulded as if from clay, as in the heavy oil of The Edge of the Plain (1926), where the clouds are like zeppelins. If John’s landscapes always seem emphatically rooted in the sense of place, Paul’s strain towards a transcendence of it. Early in their lives, the elder brother had always led the way.

Paul went to the Slade to study painting; John drifted and dabbled after school. When Paul joined up in 1916, he was immediately commissioned as an officer; John was a private who rose to the rank of sergeant, excelling as a soldier, but never making it past the officers’ selection board. Paul was in the trenches for eight weeks and saw little action, eventually falling down a hole one night and copping a classic ‘blighty’ injury (a cracked rib), less than two weeks before most of his fellow officers were killed in an assault. John was constantly involved in action, leading 10-man bombing parties and remaining committed throughout to frontline service.

In actuality, the brothers behaved more like collaborators than rivals, but the candour necessitated by mutual respect must have caused its own tensions. As John wrote to his brother in 1917: ‘We have always liked or not liked each other’s work and it has always appeared quite simply either good or bad.’

This admirable stance makes the prosaic nature of the brothers’ final falling-out in 1945—over the division of their late stepmother’s furniture—even more dismaying. In 1918, they finally found themselves ensconced together in a large studio (an old barn) in Chalfont St Peter, Buckinghamshire, as official war artists.

They were amused that the War Office saw fit to deliver a truckload of barbed wire and other military accoutrements to ‘remind’ them of life at the front. At 6pm every evening, the brothers stopped their official artist duties and began their ‘own’ work, mainly landscape painting. It is this aspect of their output that is explored in this exhibition. Paul’s early obsession with the innate mythic qualities of landscape is evident early on. His ability to create mysterious spaces or vacuums in the hearts of his paintings can be seen, for example, in the early work Landscape with Rooks, in which the central field seems pregnant with unmeaning.

As Paul asks in one of his letters: ‘Have you ever known a place which seemed to have no beginning and no end?’ Such a question sounds like a presentiment of Robert Smithson, Spiral Jetty and the Land Art movement of the 1960s and 1970s.

Aware of the international scene, Paul developed his work assiduously, from quasi-surreal works such as The Archer (1930–42), which curator Paul Gough relates to the work of de Chirico, to the Blakean effusions of his last years when, blighted with chronic asthma, he struggled into the countryside to find inspiration for works such as the transcendental Eclipse of the Sunflower (1945). John, meanwhile, was content to live quietly in rural Essex, gardening for half the year and painting for the rest of the time, going over the same places repeatedly, seeking new meanings.

Is it unfair to suggest that he may have become complacent or stuck in a rut as a result? His successful career as an illustrator may have limited him, somehow, and made the work more superficial—although it’s a cliché to suggest that.

Perhaps the death of his brother in 1946 led to a kind of artistic atrophy. Certainly, the best of John’s works on show here, which date from the 1920s, bear comparison with Paul’s. Canvases such as The Moat, Grange Farm, Kimble, which presents a visual cacophany of trees, are both compelling and convincing, capturing the fine strangeness of an everyday landscape. The more ambitious Paul felt the critical condescension most keenly.

In 1922, he was incensed by critic Frank Rutter’s decision to condense his and John’s work into a single chapter of his book Some Contemporary Artists, complaining to his brother: ‘According to R, your art has the elements of true greatness and I am a genius fashioned by war.’ Paul’s outrage at this assumed equivalence is testament to the fact that he always found it self-evident that his own work transcended his brother’s. Paul had ambition and a gift for self-promotion, but John suffered from a very English diffidence, perhaps exacerbated by bouts of depressive illness. Here, at last, their work can be assessed in a more dispassionate atmosphere.

‘Brothers in Art: John and Paul Nash’ is at the Royal West of England Academy, Queen’s Road, Bristol, until September 14 (0117–973 5129; www.rwa.org.uk). A book of the same title is published by Sansom & Company to coincide with the exhibition (£16.50)

* This article was first published in Country Life on July 30 2014

* Follow Country Life magazine on Twitter