The commonplace view that the intellectual roots of Hans J. Morgenthau’s Realist theory lie in conservative central European political traditions (such as Bismarckian Realpolitik) requires modification. The young Morgenthau was a protégé of one of Weimar Germany’s most prominent left-wing legal thinkers and barristers, Hugo Sinzheimer, a committed Social Democrat who influenced many young jurists who hoped to pursue a peaceful, legally-based transition to democratic socialism. Although Morgenthau’s biographers have acknowledged his close personal ties to Sinzheimer, they have ignored the ways in which his early work was directly influenced by Sinzheimer’s left-wing legal sociology. Morgenthau’s initial rendition of Realism took the form of a critical-minded sociology of law, inspired by Sinzheimer, which aimed to bring about fundamental reforms to the international system. Only after the demise of the democratic Left did Morgenthau begin to shed his reformist faith in the possibility of legally based global reform. His mature version of Realism can be interpreted as a response to his disillusionment with the politically progressive and socially reformist vision of law championed by the interwar democratic Left.