The law relating to international immunities is long-established and can be found in both relevant treaties and customary international law.
States generally abide by rules of regulating the conduct of diplomacy based on reciprocity, and to facilitate the regulated conduct of international affairs – it is in every state’s interests to abide by the existing rules where they can do so.
Historic examples of state abuse of the laws relating to diplomacy are rare, particularly about the exchange of diplomats, (cf the Tehran Hostages Case). Failure by a state to comply with diplomatic law is rare and usually met with condemnation.
The law relating to state immunity, on the other hand, is significantly more contested than the law relating to diplomatic immunity and is increasingly open to state challenge, as well as to challenge by other actors, e.g. about the apparent conflict that exists between the ordered conduct of international affairs and issues of the abuse of human rights.
Sovereignty is a foundation principle of international law.
Article 2(1) of the United Nations Charter is explicit that “[t]he Organization is based on the principle of the sovereign equality of all its Members”.
Similarly, Article 2(4) of the Charter prohibits “the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations”; while Article 2(7) prohibits the UN from intervening in “matters which are essentially within the domestic jurisdiction of any state” except where taking enforcement action under Chapter VII.
Nevertheless, sovereignty is a concept that if taken to its fullest extent, arguably constitutes the impossibility of international law. This point was made clearly by John Austin who wrote in 1832 that law is the command of a sovereign body backed up by sanctions. This assertion is itself highly questionable but setting this aside for the moment, it is possible to assert that there is no sovereign body in international law and there is no centralized body of law enforcement. This realization led John Austin to assert that international law is nothing more than positive morality.
As a brief but important aside for you to consider, a more recent critic of international law, Scott Veitch has taken the concept of positive morality and questioned the immorality of international law itself as a concept that is dependent on the sovereignty of states insofar as sovereignty and the protection of internal sovereign order within states somehow legitimates human suffering – something for you to consider! Veitch, S., Law and Irresponsibility: On the Legitimation of Human Suffering, (2007) 1
The ultimate response about the compatibility of sovereignty and law should be that sovereignty is not absolute. The mere existence of international law sets the limits of state sovereignty and it is apparent that the more developed international law becomes, the more that sovereignty is limited.
One of the most important manifestations of the impact of sovereignty relates to the matter of jurisdiction, that is, the power of states to make and enforce the law within their territory. The two key forms of jurisdiction are civil and criminal.
Conflicting approaches are apparent here. An absolutist (positivist) view would assert that unless international law provides a rule limiting jurisdiction then a State can exercise jurisdiction over whatever matter it likes.
This approach was the position upheld by the Permanent Court of International Justice in The Lotus Case (1927) P.C.I.J. Reports, Series A, No.10
“It does not follow that international law prohibits a state from exercising jurisdiction in its territory, in respect of any case which relates to acts which have taken place abroad, and in which it cannot rely on some permissive rule of international law… Far from laying down a general prohibition to the effect that states may not extend the application of their laws and the jurisdiction of their courts to persons, property and acts outside their territory, [international law] leaves them in this respect a wide measure of discretion which is only limited in certain cases by prohibitive rules.”
“Though it is true that in all systems of law the principle of the territorial character of criminal law is fundamental, it is equally true that all or nearly all these systems of law extend their actions to offences committed outside the territory of the State which adopts them, and they do so in ways which vary from State to State. The territoriality of criminal law, therefore, is not an absolute principle of international law and by no means coincides with territorial sovereignty … No rule of international law regarding collision cases to the effect that criminal proceedings are exclusively within the jurisdiction of the State whose flag is flown…”
This doctrine held sway at a time when sovereignty was seen as absolute. But that approach in itself is likely to cause conflicts of sovereignty often favouring the powerful.
A better approach might be one based on the power of law to regulate sovereignty insofar as international law can set the limits of a State’s jurisdiction.
See J.Brierly (1928) 44 LQR 154:
“International law did not start as the law of a society of States each of omnicompetent jurisdiction, but of States possessing a personal jurisdiction over their nationals and later acquiring a territorial jurisdiction over resident non-nationals. If it is alleged they have now acquired a measure of jurisdiction over non-resident non-nationals, a valid international custom to that effect should surely be established by those who allege it.”
This battle of sovereignties rages on. In the modern context of transnational and international criminal law more powerful states are increasingly asserting their jurisdiction over extraterritorial crimes while at the same time, an internationalist approach recognises the importance of extending jurisdiction over states that are unable to exercise their sovereignties responsibly, for example through the creation of the International Criminal Court. The difference here is between unilateralism and multilateralism – we will now consider how this process is manifesting itself.
In the case of civil law, an entire subset of private international law or conflict of laws has emerged to regulate jurisdiction based not on the sovereignty of the state but the freedom of choice of parties.
If we focus in more depth on criminal law, the key principles are the following:
(I) territorial principle
(II) nationality principle
(III) universality principle
(IV) protective principle
(V) passive personality principle
(I) Territorial Jurisdiction
Lord Macmillan in The Cristina  A.C. 485 at 496-7:
“It is an essential attribute of the sovereignty of this realm, as of all sovereign States, that it should possess jurisdiction over all persons and things within its territorial limits and in all causes civil and criminal arising within these limits”
(II) Nationality (or Personal) Jurisdiction
“The second principle is universally accepted though there are striking differences in the extent to which it is used in the different national systems.”
Introductory Comment to the Harvard Research Draft Convention on Jurisdiction concerning Crime, 1935; (1935) 29 AJIL (Supp.) 435
(III) Protective (or Security) Principle
Injury to vital interests of States
“The third [principle] is claimed by most states, regarded with misgivings in a few, and generally ranked as the basis of an auxiliary competence.” Introductory Comments to The Harvard Draft, supra
A.G. for Israel v Eichmann, supra
“… This crime deeply concerns the “vital interests” of the State of Israel, …“
(IV) Passive Personality (Nationality) Principle
Injury to a national
“The fifth [principle, here described as the fourth], asserted in some form by a considerable number of states and contested by others, is admittedly auxiliary in character and is probably not essential for any state if the ends served are adequately provided for on other principles.” Introductory Comment to The Harvard Draft, supra. The passive personality principle was omitted from the Harvard Draft. See more recently the application of the passive personality principle to cases of terrorism. But see Cassese:
Resort to the passive nationality principle is, however, particularly incongruous in the case of international crimes such as, for instance, those against humanity, that is, our sense of humanity, in other words, our concept of respect for any human beings, regardless of the nationality of the victims. As a consequence, their prosecution should not be based on the national link between the victim and the prosecuting state. (Cassese, 2003, p. 284)
(V) Universal Jurisdiction
Crime jure Gentium
“The fourth [principle] [here described as the fifth] is wide, though by no means universally, accepted as the basis of an auxiliary competence, except for the offence of piracy, concerning which it is the generally recognised principle of jurisdiction”
Introductory Comment to The Harvard Draft, supra.
War Crimes; A.G for Israel v Eichmann (1961) 36 I.L.R. 5
“the abhorrent crimes defined in this Law are not crimes under Israel law alone. These crimes, which struck at the whole of mankind and shocked the conscience of nations, are grave offences against the law of nations itself (delicta Juris Gentium). Therefore, so far from international law negating or limiting the jurisdiction of countries concerning such crimes, international law is, in the absence of an International Court, in need of the judicial and legislative organs of every country to give effect to its criminal interdictions and to bring the criminals to trial. The jurisdiction to try crimes under international law is universal.”
Genocide – Convention on the Prevention and Punishment of the Crime of Genocide 1948  ATS 2 – limited bases of jurisdiction – territoriality and nationality.
Crimes Against Humanity – see jurisprudence of IMT and ICTY/ICTR
Case Concerning the Arrest Warrant of 11 April 2000 (DRC v Belgium) ICJ Reports 3 (2002)
Returning to the issue of immunities:
Each state is deemed to have jurisdiction and sovereignty over its territory but as a result of sovereign equality and the principle of par in parem non-habet imperium, (Latin for “equals have no sovereignty over each other”), states apply immunity for foreign states from their territorial jurisdiction. This is known as state immunity. States act through individuals. The most important of these include the Head of State, Head of Government, Ministers of State, especially the Secretary of State/Foreign Minister, and other lower-ranking state officials, including ambassadors and other diplomatic officials and, to a lesser extent, consular officers.
When such officials act on behalf of the state, they have immunity that derives entirely from the immunity of the state. This is known as immunity rationale materiae. Further personal immunity (immunity rationale personae) is provided to high-ranking state officials who frequently travel abroad including the Head of State, Head of Government, Ministers of State, especially the Secretary of State/Foreign Minister primarily to ensure their dignity and to ensure their inviolability (that they not be interfered with). Most importantly for present purposes, immunity is also provided to individuals who live abroad and do so to conduct the business of the state they represent. This includes ambassadors and other diplomats, who are entitled to diplomatic immunity, and consular officials, who are entitled to consular immunity.