Paul Nash review – between dream and nightmare

Tate Britain, LondonStrangeness on the Downs meets the horrors of two world wars in the biggest show of Nash’s work in a generationOut walking, you are confronted by a long column of trees approaching across a bare winter landscape. You stop, and it seems as if they stop too. Stationed before them – the field marshal commanding his troops – stands a stone pillar topped with a pale sphere. And this sphere has a peculiarly exact echo in the white moon hanging beside it in the sky