The conclusion offers new perspectives on how after the crises of the 1930s and the even more horrific Second World War a more durable Atlantic order for the “long” 20th century could be created – an order that was founded as a western system led by the new American superpower and rested on the Marshall Plan, the European Recovery Program and the North Atlantic Treaty Organisation. Reappraising the global significance of these developments, it emphasises that what the principal American and west European decision-makers constructed was not just propelled by the escalating cold war with the Soviet Union but rather, on a deeper level, the outgrowth of longer-term learning processes: attempts to draw deeper lessons not only from the rise of National Socialism, authoritarianism and Stalinism and the Second World War but also from the earlier crises and catastrophes of the “long” 20th century, particularly the First World War and the deficient or unfinished efforts to create a modern international system in its aftermath. Finally, it reflects on the challenges of preserving a functioning and legitimate Atlantic and global order in the early 21st century.

Chapter 11 reappraises the peace conceptions and reordering strategies of Lloyd George and the other architects of the British agenda for the Paris Peace Conference. It argues that what they envisaged centred, not on containing Germany and re-establishing a workable balance of power but rather on the novel aim to create, in cooperation with the United States, a new Atlantic concert that was to stabilise a modern international equilibrium within a recast global order. It illuminates the underlying assumptions and rationales of what became an ambitious British peace programme, which included the most elaborate and influential blueprints for a League of Nations as framework for a novel, and integrative, international concert. And it highlights that British approaches to peacemaking, which were also designed to bolster the British Empire and expand British imperial influence in the Middle East, evolved and changed significantly between the armistice and Versailles as well. Finally, it analyses the extent to which Lloyd George and other key actors like Robert Cecil and Jan Christiaan Smuts had embarked on constructive learning processes – and the extent to which their evolving concepts and strategies were conducive to the creation of a durable and legitimate Atlantic and global order.

Chapter 7 offers a newly comprehensive interpretation of the political and ideological war that escalated at the heart of the First World War. It argues that at the core the war turned into a transatlantic struggle not only between war-aim agendas but indeed between competing liberal-progressive, imperialist and Bolshevik visions of peace and future order. It elucidates the unprecedented scope of this struggle by examining not only the aims and conceptions of the different wartime governments and leaders like Wilson, Lloyd George, Ludendorff and Lenin but also the contributions that intellectuals, opinion-makers and other non-governmental actors and associations on both sides of the trenches made to what became the greatest war for “national minds” and “world opinion” in history (up until then). And it brings out the far-reaching consequences this struggle had, both for peacemaking after the war and in the longer term. The analysis emphasises that it catalysed or brought to the fore formative ideas and ideologies of international and domestic-political order for the remainder of the “long” 20th century, including notions of self-determination and universal but hierarchical democratisation, ideas for a modern league of nations and competing blueprints for an internationalist system of communist states.

Chapter 10 reappraises the evolving plans and visions for a League of Nations and a new, progressive international order that were advanced by Woodrow Wilson and those who came to advise the American president and contribute to the American peace agenda that was presented at the Paris Peace Conference. It reinterprets Wilson’s core aspiration as, essentially, the pursuit of a new Atlantic order – rather than a “new world order”. And it not only analyses the underlying assumptions and maxims of the peace programme that he and his core advisers elaborated after the end of the Great War – and the crucial changes they made to this programme and their approaches to peacemaking during the critical phase between the armistice and the peace negotiations at Versailles. It also evaluates how far Wilson and his advisers had drawn deeper lessons from the war – and how far the president’s reorientated ideas and strategies for a “peace to end all wars” actually met essential requirements that had to be fulfilled to create a durable and legitimate postwar order in and beyond the newly vital transatlantic sphere.

Chapter 24 shows how the Atlantic peace order of the 1920s was consolidated and developed to a remarkable degree in the era of London and Locarno (1926–1929), which witnessed a revitalisation of the League of Nations after Germany’s entry in 1926, advances towards an Atlantic concert which found expression in the war-renunciation pact of 1928, and concerted efforts to pursue a pacification and accommodation process between the western powers – notably France – and Germany. It analyses the forces and reorientations that buttressed this incipient transformation and examines its impact on Europe, the United States and the world. Yet it also, finally, sheds new light on the question of why the nascent Pax Atlantica of the post-World War I era disintegrated so rapidly under the impact of the World Economic Crisis.

Chapter 23 shows how in the wake of the Ruhr crisis a further transformation of European and transatlantic politics was initiated, which led to significant reforms of the Versailles system and the construction of a more sustainable Atlantic peace system in the mid-1920s. It brings out that this system came to be premised on the watershed settlements of the London reparations conference of 1924 and the Locarno security pact of 1925. And it argues that what made these advances possible were constructive learning processes both on the part of the victors and on the part of the vanquished of the Great War. It highlights that these processes gave rise to the formation of a novel European concert, which began to integrate Weimar Germany in a reconfigured Atlantic order, supported by an informal American hegemon, that began to stabilise not only western but also eastern Europe.

Chapter 12 reassesses the peace conceptions and reordering and security policies of Clemenceau and the other authors of the French programme for the Paris Peace Conference. It argues that while their key aim undoubtedly was to secure France against what they saw as the critical threat of renewed German aggression they came to pursue new, essentially transatlantic strategies to this end, which came to concentrate on efforts to establish, in cooperation with Britain and the United States, a new Atlantic alliance and security community. It illuminates the underlying assumptions and rationales of the ambitious French agenda, showing that it too was significantly reorientated between the armistice and Versailles. Finally, it explores how far Clemenceau, his main adviser André Tardieu and other French policymakers had drawn constructive lessons and consequences from the catastrophe of the war – and how far their aims and strategies, which focused on containing and initially isolating postwar Germany, indeed opened up realistic perspectives of securing peace and creating a durable Atlantic and global order.