Everyone suddenly burst out singing;
And I was filled with such delight
As prisoned birds must find in freedom,
Winging wildly across the white
Sassoon wrote that this poem, rather like its opening line, burst onto the page in a strange fugue of inspiration.  In his own words , from his memoir  Siegfried’s Journey : 
 
 One evening in the middle of April I had an experience which seems worth describing for those who are interested in methods of poetic production. It was a sultry spring night. I was feeling dull-minded and depressed, for no explainable reason. After sitting lethargically in the ground-floor room for about three hours after dinner, I came to the conclusion that there was nothing for it but to take my useless brain to bed. On my way from the arm-chair to the door I stood by the writing-table. A few words had floated into my head as though from nowhere… I picked up a pencil and wrote the words on a sheet of note-paper. Without sitting down, I added a second line. It was as if I were remembering rather than thinking. In this mindless, recollecting manner I wrote down my poem in a few minutes. When it was finished I read it through, with no sense of elation, merely wondering how I had come to be writing a poem when feeling so stupid. I then went heavily upstairs and fell asleep without thinking about it again… 
 The poem was ‘Everyone Sang,’ which has since become a stock anthology piece. No one has ever said a word against it, and it is now almost as well known as Yeats’s ‘Innisfree.’ What I have been unable to understand is that there was no apparent mental process during its composition. Many of my shorter poems have been written with a sense of emotional release and then perfected by revision — often after being put away for a long time. Others have been produced by mental concentration and word seeking which lasted two or three hours. But there was usually a feeling of having said what I wanted to with directness and finality. ‘Everyone Sang’ was composed without emotion, and needed no alteration afterwards. Its rather free form was spontaneous and unlike any other poem I have written. I wasn’t aware of any technical contriving. Yet it was essentially an expression of release, and signified a thankfulness for liberation from the war years which came to the surface with the advent of spring. 

Everyone Sang