Until approximately two centuries ago criminal jurisdiction over military personnel stationed abroad did not raise any issues in international State practice. Therefore, no case law on the issue was to be found until 24 February 1812 when the US Supreme Court in The Exchange v. McFaddon case touched upon the issue for the first time. This case serves as the point of departure for this book. In The Exchange v. McFaddon case Chief Justice Marshall considered that granting a free passage to foreign military forces implied a waiver of the host State’s jurisdiction over the visiting forces and permitted its commander to exercise his jurisdiction. In later case law this consideration was extended from the transit of forces to the presence of foreign forces and was to play a key role in discussions and reflections on criminal jurisdiction over military personnel for a long time.

Chief Justice Marshall stated that immunity could only exist, if the host State had consented to the presence of the foreign forces. This is a legitimate point when viewed from an international law perspective, as the entry of armed forces into the territory of other States without their consent or any other international legal basis, such as a mandate of the UN Security Council, may constitute an international wrongful act. Obviously, in similar situations it cannot be assumed that States have waived their jurisdiction. Should incursions of foreign armed forces lead to an armed conflict, the situation would further develop and the presence of foreign forces would have to be considered in accordance with the law of armed conflict.

Although the nineteenth century literature and case law generally reflected the ideas described above, the relation between sending States and their armed forces was also emphasised. It was understood that immunity only applied to military personnel as members of an organised military unit, but not to individual soldiers who committed an offence while off-duty outside the military premises, such as a garrison or a ship. The idea seemed to be that only in these conditions was the military commander in the position to exercise his direct control over the force by taking disciplinary action.

This approach also reflects the doctrine of absolute State immunity. As States act through their organs and State officials, only their official acts are attributable to the States and, therefore, State officials benefit from the immunity of the State. What the literature and case law make clear is that in respect of military personnel the relation between their acts and the State is key and not just the military status of a person as such.

Since the nineteenth century judges have accepted restrictions to immunity of States. This process of gradual modification of the absolute doctrine of State immunity evolved in the field of civil law, especially in respect of adjudicative jurisdiction, while immunity from enforcement jurisdiction largely remained untouched. The latter also applies to criminal immunity of States, which is without question absolute: a State cannot be held criminally responsible for its acts before a foreign court.

In the field of criminal law high-ranking State officials, such as Heads of State, Heads of Government and Ministers of Foreign Affairs, and high-ranking staff members of international organisations, are entitled to full immunity. They need to carry out their tasks undisturbed and without interference of other States to ensure the orderly conduct of international relations. Therefore, they are fully exempted from the foreign courts’ exercise of criminal jurisdiction. A similar degree of immunity may apply to high-ranking military officers, such as the commander of an international peacekeeping force led by the UN. His claim to immunity is, by the way, not based on his military status, but on his specific international position.

The position of lower-ranking State officials and staff members of international organisations normally does not require that they enjoy full immunity under all circumstances. This category of officials, therefore, can be subject to the criminal jurisdiction of the host State, if offences are committed outside their official duties. International law does not distinguish between the various backgrounds of these officials and does not make a specific exemption for military personnel, either. Therefore, military personnel are also entitled to functional immunity. In general, this rule applies to all members of the sending States’ armed forces and may also apply, under specific circumstances, to military personnel who serve an international organisation in a personal capacity, for example, as a UN expert on mission. Viewed from this perspective, it is no surprise that the historical analysis shows that foreign forces hardly ever enjoy a lower level of immunity than functional immunity from host State jurisdiction. The analysis further demonstrates that there is no uniform immunity rule applying to all military personnel stationed abroad, as the frameworks within which armed forces operate abroad are too diverse.

In respect of the absence of a uniform immunity rule, military operational law can provide additional points of reference. Viewed from this particular legal perspective, SOFAs, as part of the jus in praesentia, do not stand alone, as they build on the jus ad praesentiam that encompasses the general and specific legal bases for foreign military presence. Jus ad praesentiam and jus in praesentia together constitute the law of visiting forces, which is part and parcel of the legal framework of military operations and is as such an essential part of military operational law. This makes SOFAs a commanders’ tool when preparing and executing their missions, as SOFAs expedite entry into host State territory and facilitate day-to-day operations. From the perspective of military operational law, consent together with the legal bases for foreign military presence have great impact on the content of SOFAs, including the key provision on criminal jurisdiction.

Independently of the legal bases and based on the general principles of military operations, sending States will seek immunity for their deployed armed forces as protection against foreign legal systems. Often this endeavour is prompted by unfamiliarity with those legal systems or the plain fact that the system is different from the familiar national system. Sometimes, host State legal systems have not fully been developed yet, or do not function properly due to a crisis situation. Also, sending States may fear that host State criminal justice authorities will be prejudiced against foreign soldiers, which may jeopardise a fair trial.

Immunity from the host States’ criminal jurisdiction is without prejudice to the sending States’ control over their forces and must not lead to impunity. It is of paramount importance that sending States can prosecute military personnel who enjoy immunity abroad and to that end criminal law of the sending States must be applicable to them. Many States have adopted specific provisions thereto. In my opinion, extraterritorial application of the sending States’ criminal law is a prerequisite for granting a level of immunity beyond functional immunity.

The historical analysis of Part I has explored to what extent criminal jurisdiction over military personnel stationed abroad could be explained from three different frameworks: armed conflict, crisis management operations and international military cooperation, while taking into account the circumstances under which the forces are present abroad, the interests of the States involved and their mutual relations. In Part III military operational law considerations were added. The following subsections merge all elements for each framework, respectively, allied forces on co-belligerent territory during armed conflict (Sect. 14.2.4.1), participation in crisis management operations (Sect. 14.2.4.2), and international military cooperation during peacetime (Sect. 14.2.4.3).