Chapter 9 details Tinbergen’s activities during World War II when he was working at the Central Bureau of Statistics. It uncovers crucial details about his relationship to the German occupiers and the peculiar deal he struck with them, in particular, Ernst Wagemann, to maintain some degree of independence for the institute. It also seeks to understand his attitude toward fascism, which he strongly condemned at a personal level, but whose economic policies he repeatedly praised in his writings, both before and during the war. The restrictions imposed on research at the CBS during the war meant that Tinbergen could not continue his studies into the business cycle, which had been declared a relic of the past by the Germans. In response to these restrictions Tinbergen wrote some of his more systematic work in economic theory and economic growth, further removed from policy. What is most striking is that precisely during the turbulent 1930s and 1940s a notion of an autonomous economic system emerged in his work. This detachment from politics and society is analyzed in detail. The chapter closes with a discussion of his efforts during the early Reconstruction years as director of the newly founded Central Planning Bureau.