This study reflects on Arthur Cecil Pigou's role in public debate during the initial phase of the First World War over whether Britain should negotiate a peace treaty with Germany. Its main goal is to provide evidence that the "Cambridge Professor"framed his approach to this highly controversial issue from theoretical propositions on trade, industrial peace, and welfare that he had developed in previous works. After reviewing his contributions on these subjects, Pigou's letter to The Nation in early 1915, suggesting an open move by the Allies towards an honorable peace with Germany, is presented along with his more elaborate thoughts on this same theme put down in a private manuscript. The negative reactions to Pigou's letter are then scrutinized, particularly the fierce editorial published by The Morning Post. A subsequent version of Pigou's plea for peace, delivered in his London speech late in 1915, is detailed, listing the essential conditions for a successful conclusion of the conflict. To come full circle, the paper recapitulates Pigou's postwar considerations on diplomacy, free trade, and colonialism. The concluding remarks bring together the theoretical and applied branches of Pigou's thoughts on war and peace.