Karl Barth and Emil Brunner rose to theological prominence in the 1920s as leading spokesmen for the new 'dialectical theology' movement. Thus, many were surprised by Barth's vehement rejection of Brunner's theology only a decade later in their famous 1934 'natural theology' debate. For the past sixty-five years, there has been little investigation into the root causes of their parting of the ways. This book is a historical and theological analysis of the coming together and falling apart of the Barth-Brunner alliance in the years 1916-1936. Through a close study of their writings and their recently published correspondence, the radical and powerful nature of Barth's theology is demonstrated. For what separated Brunner from Barth is what separates Barth from every theologian.