This chapter looking at the material presented about the post-WWII, period seeks to identify categories of incentives behind uses of interstate force: struggle to end colonialism, claims of historical rights, spread of ideology, religion, or culture, need for natural resources, wishes to prevent nuclear proliferation. Hurt pride and the aspiration for hegemony are also discussed as incentives to the use of force by the Soviet Union/Russia, US and China. Colliding aspirations for hegemony are noted as special risks of clashes and conflict

In all human societies, whether primitive or advanced, there have been legal norms rejecting the use of force. Norms have emerged from custom. Written law may have resulted from the codification and development of customary rules. This happened in Sweden around 1200 A.D. when a central power succeeded in wielding control and secured the unification and development of regional norms into common rules that became binding on all. Over time majority decisions became the established mode of rules adoption. In the international community, no central power has attained control and there has been no legislature adopting rules binding on all states. At Westphalia, in 1648, after the thirty years war, and in Vienna, in 1815, after the Napoleonic wars, the great powers victors felt a responsibility to design a peaceful order. However, it was only through the joint adoption of the Covenant of the League of Nations and the Charter of the UN that the states of the world agreed on binding themselves under legal norms prohibiting the use of interstate use of force.

Traditional wars with the aim to conquer territory have become rare after WWII. The Russian invasion of Ukraine departs from the trend but does not fundamentally alter it. This book examines the incentives, and restraints on the interstate use of force. Theories on the causes of war are discussed and a description of post-WWII conflicts and uses of force is offered. Discussion of restraints will focus on the role of military – including nuclear – deterrence, on the effects of conciliation, diplomacy, and disarmament and on the effects of restricting legal rules, notably the UN Charter.

Analysis is offered the UN Charter articles, notably Art. 2:4, that relate to the use of force. The Friendly Relations Declaration adopted by the General Assembly in 1970 is a useful tool of interpretation. The forcible seizure of territory is found to be a primary objective of the ban on the use of force. Post WWII, such seizures have been uncommon, but a few are cited. Interventions are of different kinds and many – that contain elements of force – fall under the prohibition in Art. 2:4. The term ‘force ‘does not cover ‘economic pressures’ but may cover various kinds of cyber-attacks. What the ban on ‘threat’ of force covers has not yet been authoritatively clarified. As only force ‘in international relations’ are covered, the article does not restrict governments’ use of force inside their own territories but it does prohibit foreign interventions supporting rebels. The case of the civil war in Syria and the interventions in it is discussed.

The international community has come a long way to develop a fabric of rules by means of treaties. While the effectiveness of this system generally is high, it is not easy to assess the impact of the handful of rules of the UN Charter that outlaw the interstate use of force. The absence of armed conflicts between great powers or other signs of restraints on the use of force may have been the result of other factors than the ban in the Charter, like fear of initiating nuclear war or fear of the economic cost of ruptures. The rule allowing force in self-defence only when an armed attack occurs has been under strain. Interventions have been undertaken, for instance to remove governments or to forestall nuclear proliferation. While unable to take enforcement actions, majorities in the UN General Assembly have been remarkably firm in an orthodox affirmation of the rules. Through interpretation there has also been some mitigation in terse rules. Thus, the prohibition of the interstate use of ‘force’ has not been taken to cover economic pressure and the right to self-defence ‘if an armed attack occurs’ may have regard to both past attacks and attacks deemed ‘imminent’ – but not to ‘anticipated’ self-defence.

UN Charter Art.33 points to means such as negotiation, mediation and arbitration to settle disputes and to avoid the risk of use of force. Judgments of the International Court of Justice are of great value also to further develop international law. However, courts are hardly ever employed to save peace by the judicial settlement of big disputes. Following the precedent of the Nuremberg tribunal, special tribunals have been set up to try war crimes. Even more important, an international criminal court has been established. Fact-finding (termed ‘enquiry’) has achieved a prominent role both to solve and to prevent conflicts. Thus, the safeguards system of the IAEA using inspectors, cameras and satellites and environmental sampling continuously verifies that no fissionable material is diverted away from peaceful uses. With much fake news around, impartial professional inspection, monitoring and reporting – for instance, about the use or possession of chemical weapons – has become more important. Diplomacy seeks to secure or create peaceful relations by agreement, precluding or removing the threat or use of force. Examples are given of successes of diplomacy to preserve peace as well as of failures and of talks going forever without result.

This chapter gives brief descriptions of Post-WWII interstate conflicts linked to the main phases in great power tensions: the bipolar Cold War, with Korean war 1950, the Viet Nam war 1965, Interventions, Cuban Missile Crisis 1962, the Détente and unipolar conflict resolving world that followed the break-up of the Soviet Union and the multipolar world that is emerging thereafter with terrorism, Russian intervention in Georgia and war in Ukraine and sharp tensions between the Koreas and in the East and South China Seas.

A farewell is given to something that is leaving. After thousands of years of freedom for nations to go to war, new proximity, interdependence, risks of nuclear war have led them to exercise restraints and to commit themselves to the UN Charter and the requirement to abandon interstate uses of force. This book concludes that while states will continue as always to compete, the great powers are saying farewell to direct interstate wars. For over 75 years there have been no wars between them and no nuclear weapon has been used. The Charter as rule-based international order has often been violated and the veto has often stood in the way of action. Yet, the General Assembly has asserted the order and declared that it does not recognize illegal annexations. The nuclear arsenals remain and pose existential risks to humanity. At the same time, the fear that they would be used in second strikes makes it implausible that any nuclear-armed states would initiate hostilities that could risk leading to nuclear war. Russia’s aggression in Ukraine is seen as out of tune with the twenty-first century – an aberration. The growing interdependence of states is creating restraints against causing ruptures and, at the same time, enables states to use crippling economic measures as substitutes for the use of force. In 1962, the Cuban missile crisis shocked the parties and prompted them to use diplomacy to avert the acute risk of nuclear war. Today, one may speculate if the war in Ukraine and threats to the human environment might shock nations to turn to diplomacy and disarmament and switch to the defence of the threatened human environment a major part of the some 2 trillion dollars that they now spend annually to defend themselves against each other. For this to become reality, an engaged public mind would be as important as it was against slavery and nuclear weapons.

UN Charter Art. 2:4 aims to protect states from forcible encroachments by other states, but it does not stand in the way of the Security Council taking or authorizing states to take enforcement actions under Art. 42 and did so in the case of Korea 1950 and Iraq in 1991.It may take or authorize enforcement actions – even inside states – under Art.2:7 and 42. It may also under Art. 53 authorize the use of force by regional organizations – and has done so. The Responsibility to Protect (R2P) doctrine envisages the use of force to remedy extreme internal situations, such as genocide or massacres, but only within the rules cited requiring approval by the Security Council. The veto is often criticized as hindering action by the Council. It may, indeed, be excessively used but may sometimes be only a signal from one of the permanent members that it may be ready to use its power to resist an action proposed. Post WWII, force has been used by states – but only rarely – to acquire territory while ignoring the Security Council, notably by North Korea in 1950, Iraq against Iran in 1980 and against Kuwait in 1990, Russia against Ukraine in 2014 and 2022.

Disarmament – in the sense of eliminating weapons – has not been a success as a means of restraining the use of force between states. Significant discarding of conventional weapons took place in Europe after the Cold War under an agreement (CFE) between NATO and the Warsaw Pact. Large scrapping of whole categories of tactical nuclear weapons also occurred through parallel commitments under the so-called Presidential Initiative by Presidents Bush and Gorbachev. However, the subject of ‘general and complete disarmament’ that has long been on the international agenda, has hardly even been taken seriously. And while the non-nuclear weapon states allowed the Non-Proliferation Treaty of 1970 to be prolonged indefinitely, the five nuclear weapons states parties have failed to live up to their obligation to agree on disarmament. A significant failure is also that the US and some other nuclear weapon states have failed to ratify the Comprehensive Test Ban Treaty concluded in 1996. The US and the Soviet Union/Russia have sought – not disarmament but – important strategic stability through ‘arms control’ in bilateral agreements. In 2021, they prolonged the START agreement setting limits on the number of American and Russian strategic nuclear weapons and carriers and providing for important mutual verification.

The high ambition of the UN Charter Art. 2:4 to prevent interstate use of force is confirmed by the narrow right to individual or collective self-defence given to states under Art.51 ‘if an armed attack occurs’. States have many times invoked ‘self-defence’ as default justification of interventions and sometimes sought room for interpretations giving them more elbow room to use force in response to hostile acts of gravity. The general right of reprisal has been rejected, but in cases of grave or repeated terrorist attacks responses by armed force have not been criticized even though taken much after the attacks. A right has also been recognized to use force in self-defence where armed attacks are ‘imminent’ (pre-emption), but no such right has been acknowledged where the threat of attack is not imminent (‘anticipatory self-defence’ or ‘preventive’ use of force).

While the focus of the book is on the interstate use of force post-WWII, this chapter holds a rear mirror and offers a perspective of evolution of restraints that started long before states came into being. It recounts how human societies over the centuries became states free from widespread internal use of armed force and how great powers sought to avoid major armed conflicts through policies of balance of power and multilateral conferences. It describes how they developed common rules by concluding conventions and built institutions such as the League of Nations and the United Nations to create a rule-based order and mechanisms and methods to prevent the interstate use of force.

Most states are of the view that military forces are needed and effective as means to reduce the risk of foreign attacks. They must note that some of today’s political and military realities are very different from those that existed up to the end of WWII. Border lines and land have lost some importance. The United Nations is a new factor but one that has not reduced the importance of national defence. Underestimating the need may be fatal, but exaggerating it means taking resources from more deserving needs. A table is offered of the military expenses of several states. Military budgets are currently (2023) sharply rising in many countries. The NATO alliance has set a minimum target of 2 % of GDP for national military budgets of their members. The US has long declared a policy of ‘all sector dominance’ and has by far the largest military budget. To deter the threat of nuclear and other non-conventional weapons raises intractable new issues. The three major nuclear weapon states seek to make sure that they will not ever lose the capacity to deter nuclear attacks by a second strike that would bring pain calculated to be unacceptable.

Biological and chemical weapons are banned by treaty and attract less interest by the military than do nuclear and other modern means like cyber, space action and artificial intelligence. The number of nuclear weapons has gone down but there is no sign of their elimination through acceptance of the Treaty Prohibiting Nuclear Weapons (TPNW) or otherwise, nor is there any prospect of early common commitment to ‘non-first use’. Rather, at least the major nuclear-armed states regard their ability to inflict devastating second nuclear strikes as indispensable to deter any first strike and the ‘nuclear posture reviews of the US and Russia retain a good deal of freedom of action. We cannot at present see signs of zero nuclear and must agree with the conclusion that so long as nuclear weapons exist, there remains a risk of use – through misunderstandings or technical errors. We may also conclude that as cyber, space and other new means of struggle have become available, and capable of escalating, rendering conflicts increasingly unpredictable. It becomes implausible that any civilian or military leadership would allow itself to initiate or slide into conflict. It seems likely that they would choose intense competition by means other than force, notably economic and financial.