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.

Chapter 5 illuminates systematically – in a European, transatlantic and global context – not only the prehistory of the July crisis of 1914 but also the decisive longer-term changes in the international system and ground-rules and assumptions of international politics that led to the outbreak of the Great War. Challenging long-standing interpretations as well as the recently influential notion that European leaders acted like “sleepwalkers”, it underscores that what really proved decisive were two crucial developments: on one level, the final demise of the European concert as a key mechanism for peaceful conflict-resolution and the emergence of two antagonistic alliance blocs; and, on a more fundamental level, processes that led those who made the key decisions in and before 1914 to “unlearn” what was required, not merely to defuse continual crises at the eleventh hour but actually to manage the core systemic challenges of the age of imperialism and preserve peace more effectively. It thus seeks to show in a new way why by 1914 the escalation of a general conflict, which then widened into the First World War, had become all but unavoidable.

Chapter 3 re-examines the “ascent” of the United States within the 19th century’s Eurocentric international order, retracing its special path from a fledgling and vulnerable republic to the status of an exceptional and exceptionalist world power. It focuses on the evolution of American ideas and ideologies in the sphere of international affairs, the rise of distinctive forms of US imperialism and unilateralism, and the emergence of core maxims of US international conduct such as those embodied in the ever more expansively defined Monroe Doctrine and the Open Door Doctrine. It then casts new light on ephemeral aspirations to establish a modern Atlantic order of empires – led by the United States and the British Empire – that were pursued after the Spanish-American war of 1898 and in the era of Theodore Roosevelt.

Chapter 8 reassesses the question of why it ultimately proved impossible to end the Great War earlier and through a “peace without victory” along the lines the American president Wilson and other proponents of a negotiated settlement proposed. Then it re-examines how the war actually came to an end in the west. And analyses the making and consequences of the armistice that was finally concluded on 11 November 1918, highlighting that it only provided frail foundations and parameters for a modern Atlantic peace.

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 20 sheds new light on what arguably was the crucial problem of the peacemaking process of 1919 – the fact that ultimately the peace settlement and the terms and rules of the new order were not negotiated between the victors and the vanquished but imposed by the western powers, above all on the fledgling Weimar Republic. It shows that what led to this outcome were not only the unprecedented transatlantic complexities Wilson and the principal British and French “peacemakers” faced when trying to hammer out the fundamental parameters of the peace and postwar order, and to reconcile their often all but irreconcilable interests, aims and ideological priorities. As it underscores, the imposed peace of 1919 was also the outgrowth of underlying hierarchical assumptions the victors shared about their “right” to dictate terms to the defeated German power and to place it on probation. Also analysing the strategies that leading German policymakers like Brockdorff-Rantzau and Matthias Erzberger pursued to open the door to a negotiated peace it finally reappraises the eventual political-cum-ideological battle between the victors and the vanquished that escalated in 1919 and analysis the destructive consequences of the peace settlement’s ultimate imposition.

Chapter 9 provides the most comprehensive appraisal yet not just of the unparalleled range of pressing problems but also of the underlying structural and systemic challenges that the principal peacemakers confronted in the aftermath of the Great War. In particular, it throws into relief that by the end of 1918 not only eastern Europe had to be fundamentally reorganised after the collapse of the eastern empires and, crucially, the cardinal German question had to be settled in a new way but also, and above all, the need had arisen to create, for the first time, a functioning Atlantic order that included both the old and new European states and the newly pivotal American power. At the same time, it argues that the war had radically changed the international, national and transnational conditions in which a peace settlement had to be negotiated and cardinal decisions about the shape of the postwar order had to be made. And it shows that the unprecedented catastrophe had given rise to unprecedentedly far-reaching – and conflicting – expectations on all sides of a “peace to end all wars” that was to be forged and compensate the different societies and populations for the unprecedented sacrifices they had made.

Chapter 19 focuses on the political and moral stakes of one of the most contentious questions of the peace conference: on what grounds Germany was to pay reparations and how high the reparation claims of the victors were to be. It not only demonstrates how intricately the indemnity problem was linked with the fundamental question of who bore responsibility for the Great War and all the casualties and destruction it had caused, eventually leading to a clash between western claims of Germany’s “war guilt” and German efforts to refute them. Placing this problem in a transatlantic context, it also emphasises that the reparations conundrum was inseparable from the tectonic changes the war brought in the financial and economic spheres, especially America’s ascent to the status of the world’s pre-eminent economic and financial power and the massive indebtedness of Britain and France to the new “world creditor”. It thus casts fresh light on the question of why it proved impossible to negotiate a “rational” and mutually acceptable reparations settlement in 1919. And it reappraises why only limited advances towards a new financial and economic order and effective postwar reconstruction could be made. Finally, it highlights the far-reaching political consequences this had.

Chapter 2 illuminates the transformation of the European and global international system in the first decades of the “long” 20th century (1860–1914). It analyses how the turn towards ever more uncompromising power politics, the emergence of modern states and the intensification of ever more unlimited imperialist competition between older and aspiring world powers – essentially, the European great powers, the United States and Japan – came to recast Europe and the world. It throws into relief how this competition and the rise of dominant imperialist, militarist and “civilisational Darwinist” doctrines and assumptions not only led to the creation of a new global hierarchy characterised by unprecedented inequalities between imperial world states, smaller states and those who were subjected to different forms of informal imperialist domination and formal colonisation. And it offers new perspectives on how the confluence of European balance-of-power practices and escalating global rivalries successively corroded international peace.