WWI had a transformative effect on the lives and ideas of both Carl Jung and Wilfred Bion. Both suffered intense and life-changing experiences, which they carried with them for the rest of their lives. For Jung, living in neutral Switzerland, the febrile tension of the war emerged in a stream of archetypal imagery, while his daily life interspersed periods of analytic practice and family existence with periods of military service. For Bion, as a tank commander in the British Army, the gritty reality of mud, confusion and shell fire were his daily fare. The impact of the war left its mark on both of them, forging their emotions, their thinking and their theories. In this sense, WWI was a crucible, shaping their experience and their future conceptualizations. This paper reflects on the experiences of both men and on the consequences of these experiences and contemplates the lessons that may be learned from history today.