During the First World War the focus of Frank's written output shifted towards topics in international and maritime law. Of particular political interest was his justification, published in 1915, of the violation of Belgian neutrality by the Imperial Army in August 1914. After the war reached its dénouement Frank teamed up with to deliver and publish an opinion repudiating as unconstitutional moves to extradite the exiled emperor from his sanctuary in the countryside outside Utrecht. Many years later, in 1929, Reinhard Frank was asked about his own political perspective during the turbulent post-war period that gave birth to the German republic. He recalled that he would have considered himself as democratic, liberal, and of the "political left" until the "revolution with its anti-patriotic undertones pushed him sharply towards the right".