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 22 elucidates how a consolidation of the truncated order of Versailles was first hampered by the divergent longer-term outlooks of the victors and later decisively affected by Wilson’s defeat in the battle over the ratification of the Treaty of Versailles and the Covenant of the League of Nations in the American Senate. It then analyses systematically, and in a global context, how in the aftermath of the American withdrawal from it the crisis of the Versailles system escalated into a full-blown conflict in postwar Europe, which culminated in the transformative Franco-German Ruhr crisis of 1923.

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 15 offers new perspectives on the formative struggle to establish the League of Nations as an effective international organisation at the heart of the postwar order. It argues that in spite of the global conceptions they advanced its key architects intended the League to become the superstructure of a new transatlantic international order and security architecture. It analyses how far it was possible to find common ground between the most influential American and British blueprints for an integrative League and the markedly different French plans for an institution of the victors whose main purpose was supposed to be to protect France and constrain Germany. And it illuminates why ultimately the League of Nations came to be founded as a truncated organisation dominated by the principal victors of the Great War and initially excluding the vanquished, which were required to undergo a period of probation to become eligible for membership. Finally, it explains the far-reaching consequences this had and examines how far the League nonetheless had the potential to become the essential framework of a modern Atlantic and global order over time.

Chapter 17 explores how far the new Atlantic order that began to take shape in 1919 could be extended to the most unsettled region after the war: the post-imperial terrain of central and eastern Europe. It reassesses how the victors sought to balance in different ways newly prominent claims of national self-determination and fundamental strategic considerations in their efforts to create a stable system of states in this region – and of how they interacted with the representatives of the numerous east European national causes. While also analysing the Czechoslovakian settlement it then focuses on the victors’ attempts to “solve” the most critical problems in this context, the Polish and the Polish-German questions. And it underscores how extremely difficult it proved to establish a viable Polish nation-state that was not from the outset divided from its more powerful German neighbour by conflicts over contested borders and minority problems. More broadly, it shows how challenging it was to establish effective mechanisms to protect the rights of German, Jewish and other minorities in the new and very heterogeneous east European states. And it elucidates that the western powers’s capacity to forge a durable new order reached distinctive limits in the east.

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 18 analyses how the principal western policy- and decision-makers of 1919 sought to deal with the Bolshevik challenge or, more precisely, the political challenge posed by Lenin’s regime and the political and ideological challenge posed by what they regarded as the threat of a transnational spread of Bolshevism across and beyond Europe. It also reassesses the overall significance of the Bolshevik threat for the making of the nascent Atlantic order after the First World War, underscoring that it affected the peacemaking process but was not as decisive as generations of cold war historians have claimed and highlighting that this process was by no means shaped by a struggle between Wilsonianism and Leninism. It then examines how Lenin, Trotsky and other leading Bolsheviks interpreted the western powers’ pursuits and sought to expand the communist revolution westwards, also through the Communist International. And, finally, it illuminates how difficult it proved for Wilson, Lloyd George, Clemenceau and their advisers to agree on a common approach both towards the Russian civil war and the Bolshevik regime – and how they eventually concentrated on isolating it and preventing a German-Bolshevik alliance, ensuring that subsequently the Soviet Union would remain outside the nascent Atlantic system.

Chapter 14 presents a new interpretation of the peacemaking and reordering process that unfolded at the Paris Peace Conference of 1919. It argues that it was not only the most complex process of its kind in history but also, at the core, a process that was dominated by the struggle to negotiate the underpinnings and ground-rules of a new Atlantic order – which in turn had far-reaching global repercussions. Taking into account the unprecedented multiplicity of governmental and non-governmental actors who tried to influence this process in and beyond Paris it sheds new light on how the peace negotiations ultimately came to be shaped by the interests, concepts and strategies of those who led and represented the most powerful states after the war: Wilson, Lloyd George and Clemenceau – and their main advisers. And it opens up new perspectives on why the first truly modern peacemaking process remained in crucial respects incomplete. It shows that while the principal “peacemakers” began to learn how to forge complex compromises under the challenging conditions of 1919 what they ultimately managed to negotiate could not lay firm and legitimate foundations for a sustainable Atlantic, and global, peace order.

Chapter 4 examines the emergence of different internationalist aspirations on both sides of the Atlantic to supersede conflict-prone imperialist power politics and to advance towards a more pacific international order in the decades before the First World War. It compares the pursuits of liberal and both centrist and more radical socialist actors, non-governmental associations and newly important transnational networks like the burgeoning pacifist movement, the Second International and, notably, the new phalanx of those who demanded that power politics should be replaced by arbitration and authoritative covenants of international law – and who paved the way for the Hague Conventions of 1899 and 1907. It reassesses not only the guiding ideas of the vanguards of such aspirations but also the actual influence they had on transatlantic and global politics in this crucial phase, seeking to offer a systematic explanation of why these counterforces failed to civilise international politics and why ultimately they could not prevent the escalatory processes that caused the catastrophe of 1914.