Traditionally, public international law has focused upon the relationship between States as the most important legal subjects within the international system. The Peace of Westphalia of 1648 is generally regarded as a milestone and as the starting point of the modern European State system. Although new legal subjects have gradually made their entry into the international legal system, today the State remains the principal bearer of rights and obligations under international law.

Initially international law was still largely European oriented. However, since the end of the nineteenth century more non-European States have entered the international system.¹ Since World War II the number of States has increased exponentially and the international legal system has acquired a truly universal character.²

State sovereignty is reflected in the concept of jurisdiction with which a State exercises its power and authority. Jurisdiction (and the exercise thereof) is an essential characteristic of a State and consists of three elementary State powers: the legislative, enforcement and adjudicative power.³ A State can decide to apply its laws to persons and activities on the basis of its legislative power, referred to as legislative or prescriptive jurisdiction. A State can use its enforcement jurisdiction to secure or compel compliance with the laws (enforcement jurisdiction). Likewise, a State has the power to subject persons and goods to the process of its courts based on its adjudicative jurisdiction. The State’s jurisdiction is primarily territorial in nature, but it can to a certain extent be exercised outside the State’s borders based on criteria such as nationality.

This chapter explores various forms of jurisdiction and their underlying principles, while addressing the question to what extent jurisdiction can be exercised extraterritorially. Starting with a description of the development of States within international law (Sect. 6.2), it is followed by a discussion of the sovereign position of a State in its relation with other States (Sect. 6.3) and ends with an analysis of the extraterritorial exercise of jurisdiction (Sect. 6.4).

In Chap. 8, these sections serve as a basis for defining the armed forces position within the sovereign State and this chapter discusses several principles of jurisdiction that are specifically relevant to the military forces.

International law as we know it today is originally a product of reflection and developments in Europe starting in mediaeval society and continuing from the sixteenth to the eighteenth centuries. Under the influence of the Reformation and the Renaissance the idea that the State was subordinate to a higher worldly and ecclesiastic authority, represented by the Emperor and the Pope, was progressively abandoned.⁴ The idea emerged that the State should not depend on a higher order, but instead, should constitute the highest sovereign authority administering its territory. The State should be able to exercise this authority free from any external influence and in a position of equality vis-à-vis other States. This sovereign equality is captured in the maxim ‘par in parem non habet imperium’ (an equal has no power over his equal).⁵ These reflections were embedded in the previously mentioned Westphalia Peace Treaties of 1648,⁶ often considered the formal ending of the mediaeval feudal political order and the beginning of the modern State system.⁷

The sovereign State exercises exclusive authority over its own territory and population, which, however, does not imply that it does not affect the interests of other States. The use of the high seas, the establishment of diplomatic relations, or the conclusion of international treaties are examples of activities which involve other States. For this reason States agreed to certain rules, or accepted the existence of such rules, that primarily aimed to curtail sovereign power and to regulate their mutual relations.

These international legal rules were primarily the result of mutual agreements between States, either in the form of treaties or as usage and customs accepted as law. Although the national law element, such as justice and equality, played a role in the formulation of international law, these have become integrated into positive sources of legal obligations. Hence, international law is largely the product of the free will of States.⁸

When considering the concept of State, the definition often referred to is formulated in Article 1 of the Montevideo Convention ⁹:

The State as a person of international law should possess the following qualifications: (a) a permanent population; (b) a defined territory; (c) government; and (d) capacity to enter into relations.

An entity will have to fulfil the first three factual criteria to be considered a State. Fulfilment of these criteria entails the fulfilment of the fourth criterion: the State can establish relations with other States.¹⁰

With regard to the criteria of a permanent population, the Convention only requires that the population consists of people having the State’s nationality. It does not impose any condition on its size or composition. Likewise, the territory has to be defined, but borders may still be subject to discussion or dispute.¹¹ Furthermore, the mere existence of a government does not suffice; it must be capable of effectively and independently exercising its authority.

The requirements set out in Article 1 of the Montevideo Convention lack the element of sovereignty that is inherent to the existence of a State.¹² Sovereignty literally means the ‘highest power’ (suprema potestas) of a State. Originally it indicated either the absence of a worldly or an ecclesiastical authority over States. This independent position was formulated by the Permanent Court of Arbitration in the Island of Palmas case:

Sovereignty in the relations between States signifies independence. Independence in regard to a portion of the globe is the right to exercise therein, to the exclusion of any other State, the functions of a State.¹³

Sovereignty, however, does not imply that a State is above the law. International law arises from the States’ will and consent, which restricts the exercise of their sovereign powers. In its advice Customs régime between Germany and Austria (Protocol of March 19th, 1931) the Permanent Court of Arbitration considered:

Independence as thus understood is really no more than the normal conditions of States according to international law; it may also be described as sovereignty (suprema potestas), or external sovereignty, by which is meant that the State has over it no other authority than that of international law.¹⁴

From a legal point of view sovereignty implies equality of States; there is no hierarchical order in the relations between States.¹⁵ This means that a State does not have authority over any other State, because the latter is considered as its equal. This vision is included in the above-mentioned maxim par in parem non habet imperium. Viewed from other perspectives, States can differ widely in such areas as their political, economic and military capacity. These factual power relations influence their legal relations and can be reflected in, for instance, the provisions of a treaty.