Ienaga Saburo (1913-2002), was a contemporary Japanese historian known for Ienaga v. Ministry of Education, which was a thirty-two year civil trial that challenged the Japanese government claiming that their textbook screening and editing violates the constitution. Though he lost all three trials, Ienaga is widely supported by left leaning scholars for his long lasting impact on Japanese history education. The central question that I attempt to explore in this intellectual biography is how Ienaga’s early thinking defined and was defined by the rapidly changing historical moments in which he lived. In contrast to previous scholarship which reaffirms the uniqueness and consistency of Ienaga as an activist and a scholar, my thesis argues that Ienaga’s thinking changed over time in response to the intellectual and social milieu and that his “courageous and unique decision” to pursue the trials was the product of these responses. For the purposes of scope, my project covers the timeframe between 1913 and 1965, up to when Ienaga files the first court case. I examine the three major turning points in his thinking, bringing into dialogue Ienaga’s writing and the studies on social and political context of Japan in the pre-war, wartime, and post-war period. Living through the pre-war, wartime, and post-war period as a historian, Ienaga’s thinking was grounded on social sentiments and experiences prompted by anxieties and uncertainties that Japan faced as a nation. His concerns over national history and the question of how to live as a socially responsible intellectual is still relevant to this day.