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The use of force in international law
The use of force in international law

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1 History of the law on the use of force

For centuries, states have resorted to force in their international relations in order to achieve particular, desired aims. The use of violence has proved to be an accepted, although tragic in its consequences, method of resolving disputes between states. States reserved the right to wage war without any internationally agreed regulatory framework. Nevertheless, over time, the concepts of ‘just and unjust war’ emerged. The distinction between the two can be traced back to ancient Rome and the Fetials (fetiales), a group of priests who were responsible for maintaining peaceful internal and external relations and who gave rise to fetial law (ius fetiale) – religious law regarding the process of creation, interpretation and application of treaties and regulations on the declaration of war. The concept of ‘just war’ has changed over centuries (Von Elbe, 1939).

Roman law of war and peace

Deliberations about war were expected to pass through these priests, who would seek a judgment of the gods about the justice of the proposed course of action. If it was decided that a grave breach of the peace had in fact occurred, such that a just war would be warranted, the fetials would first approach the guilty city to demand redress. If, after a certain period of time, no satisfaction was given, war could begin. (...) Declarations of war were cast in form of a lawsuit, in which the verdict transmitted by the fetials was meant to decide on the question whether the war could be rightly waged. Whether or not a war should be waged (to enforce a verdict) would then be the matter for a new decision, to be rendered by the king, the senate, or even (in later periods) the entire people.

(Reichberg et al., 2006, pp. 47–8)

The doctrine of ‘just war’ was further influenced by Christian theologians such as St. Augustine and St. Thomas Aquinas, the latter famously stated in Summa Theologica that the three criteria for just war are:

  1. it should be waged by a sovereign authority (prohibition of waging a private war)
  2. it must have a just cause (punishment of wrongdoers)
  3. a just cause must be accompanied by the right intention.

Together with the rise of independent states in Europe, the doctrine began to evolve. In light of the growing number of sovereign states, wars started to be seen and defined as a state of legal affairs rather than a matter of subjective moral judgment. States no longer found themselves in a position to judge if another state’s reason for resorting to force was just or not. This approach was supported by the rise of positivism, which strongly focused on the idea of sovereignty and by the Peace of Westphalia 1648, which established the European system of the balance of power. This system survived in Europe until the beginning of the twentieth century, effectively coming to an end with the outbreak of the First World War.

In the aftermath of the First World War efforts were made to rebuild international relations between states through the establishment and operation of an international institution which would play a central role in ensuring that such acts of aggression would not occur again. The League of Nations (LON) was created in 1919 with a view to achieving this aim. Under the 1919 Covenant of the League of Nations, member states were required to submit any inter-state disputes for arbitration or seek other forms of judicial settlement at the League’s Council. However, the Covenant did not in fact revoke the right of states to resort to war, although it subjected this provision to some limitations. In 1928, another attempt at the legal regulation of the use of force was made, in the form of the General Treaty for Renunciation of War as an Instrument of National Policy, more commonly referred to as the Kellogg–Briand Pact. Parties to this treaty declared that they ‘condemn recourse to war’ and agreed to ‘renounce it, as an instrument of national policy in their relations with one another’ (Article 1).

The outbreak of the Second World War in 1939 once again marked the end of peaceful international relations. The tragic events of this international conflict led to the adoption of the Charter of the United Nations (UN Charter) in 1945 resulting in the development of a framework, aimed at regulating the use of force by members of the international community. That system remains in force.