Before good could prevail, the war had to be survived and won. For a while, Rubbra's pacifism kept him doing ‘war work’; once recruited, then promoted from private to sergeant, he ran the Army Classical Chamber Music Group, in particular a piano trio with the violinist Joshua Glazier (who had so far served as a driver) and as cellist Signalman William Pleeth. Like many other families at that time, his two sons born in the 1930s saw very little of him for several years. The trio's tours to the forces took it as far as Germany around the time the war ended. It continued after the war, with Glazier replaced briefly by Norbert Brainin just before the start of the latter's career as leader of the Amadeus String Quartet, and then over a much longer period by another Viennese-born violinist, Erich Gruenberg.

Writing his article on the Fourth Symphony, Wilfrid Mellers evidently had a good line to Rubbra. He mentioned a forthcoming further symphony in which that work's ‘subtlety and serenity of spirit’ were ‘certainly explored further in the marvellous opening’ of a piece ‘in which the voice (a mainspring of texture in music) may explicitly come into its own, for Rubbra tells me that the symphony will probably be choral, with words by Henry Vaughan’. The sketches were eventually turned into a work for mixed chorus and orchestra in which Rubbra dared to upstage his teacher Holst by setting the ecstatically mystical Vaughan text The Morning Watch.