Bion was recommissioned in the Royal Army Medical Corps as a lieutenant on 1 April 1940, and worked in a number of military hospitals including Northfield Military Hospital (Hollymoor Hospital, Birmingham) where he initiated the first Northfield Experiment. These ideas on the psychoanalysis of groups were then taken up and developed by others such as S. H. Foulkes, Rickman, Bridger, Main and Patrick De Mare. The entire group at Tavistock had in fact been taken into the army, and were working on new methods of treatment for psychiatric casualties (those suffering post-traumatic stress, or "shell shock" as it was then known.) Out of this his pioneering work in group dynamics, associated with the "Tavistock group", Bion's papers describing his work of the 1940s were compiled much later and appeared together in 1961 in his influential book, "Experiences in Groups and other papers." It was less a guide for the therapy of individuals within or by the group, than an exploration of the processes set off by the complex experience of being in a group. The book quickly became a touchstone work for applications of group theory in a wide variety of fields.