Joe asks about the making of the song.
Cannot just now track down the description, but somewhere is an recollection by Hamish's widow Katzel of him pacing up and down in their Edinburgh flat as he made the lines in late 1959 or early 1960. Timothy Neat in par two of his biography of Hamish tells how he completed the last four lines in January 1960, on a train ride from Strichen to Aberdeen, on his way back from a sympathy visit to Kenny Goldstein and family.
But as said above, the first publication note says 'May, 1960'. Certainly it was for the peace marchers - I think for the Committee of 100, who preceded the forming of the Campaign For Nuclear Disarmament [CND].