IR changed during World War II. This chapter tries to show how. First, it presents the war aims of the major belligerents – Germany, Britain, the USSR and the USA. It spends some time on Germany’s war aims before it zeroes in on the US approach to the war and its visions of the post-war peace. The main vision was liberal and internationalist in nature. It was expressed by President F.D. Roosevelt who drove a purposeful war-time diplomacy to lay down the institutional foundations for a liberal post-war order. The chapter discusses Roosevelt’s war-time conferences with Churchill and Stalin. It then takes up the criticism of the Roosevelt administration. George F. Kennan thought that it was naïve about Soviet affairs and the ‘realities of power’. Reinhold Niebuhr was critical of the ways in which the administration gullibly negotiated peace with master-Realists like Stalin. However, the sharpest attack came from Hans J. Morgenthau. In the wake of the war, he claimed that the Roosevelt administration was composed of naïve technocrats who did not know the first thing about the power-based politics among nations.