This chapter is the first of three attempts to question the assertion according to which nineteenth century authors who claimed international law ring-fenced the use of force were 'naturalists'. It is divided into four sections: The first seeks to evidence how authors that can be labelled as 'naturalists' construed the international legal order. It shows that they did not dismiss positive law but believed natural law to be normatively superior. The second investigates how this translated in terms of method for the identification and determination of custom. ‘Naturalist’ authors adopted a two-step approach which consisted in analyzing State practice in order to identify potential customary rules and then test them against ‘necessary’ principles in order to confirm their normative value. The third and fourth sections respectively try to see how this method was applied to establish the existence of a rule of non-intervention and the exceptions to said principle. Non-intervention was deduced from the natural principles of independence and equality. Practice was more extensively taken into account to establish the cases in which States could resort to force.

This chapter turns to the analysis of the practice of justifying force in the ‘semi-peripheries’, i.e., in non-‘European’ States. The purpose is to see whether legal arguments were also developed and whether they differed from the ones brought forth to justify bellicose endeavours in the ‘centre’. It focuses on four case studies: the European intervention during the Greek War of Independence (1827); the French expedition in Lebanon and Syria (1860); the western intervention during the Boxer Revolt in China (1901); the Monroe doctrine and the Roosevelt corollary with special attention to their application in Nicaragua in the early twentieth century (1909–1912). It argues that, although these interventions rarely gave rise to full-blown wars, the justifications brought forward did not substantially differ from the ones used in the ‘centre’. States, in fact, carefully developed legal arguments and evidenced how their actions were meant to redress a previous offence.

This chapter is the third and last one dedicated to the analysis of 19th State practice. It turns to precedents of justifying force in the ‘peripheries’, i.e., non-‘European’ spaces. Four case studies are more particularly analysed: building from the work of Inge Van Hulle, it looks at British colonial expansion in Western Africa; at the Anglo-Zulu War (1879); at the French Tonkin expeditions in the 1870s and 1880s; and at the United States’ annexation of Hawaii (1893–1898). It shows that even though the status of ‘peripheral’ entities in the international legal system was debated, intervening States once more took care to develop legal arguments to justify their colonial and expansionist adventures. In so doing it completes the picture of the practice of justifying force in the nineteenth century.

This chapter turns to the ‘eclectic’ literature. Its structure mimics that of the previous chapter. It starts by highlighting how ‘eclectic’ authors thought about the international legal order. Unlike naturalists, they placed natural and positive law on an equal footing. This translated into a method for custom identification and determination in which the potential contradictions between positive rules and natural principles was erased by presupposing that States’ behaviours were always guided by reason. The third and fourth sections of the chapter respectively try to see how this method was applied to establish the existence of a rule of non-intervention and the exceptions to said principle. They show, on the one hand, that the principle of non-intervention was once more deduced from the principles of independence and equality and, on the other hand, that practice was much more engaged with to establish the exception to non-intervention.

This chapter is the third and last one dedicated to the analysis of nineteenth-century doctrine. It focuses on ‘positivist’ literature and follows the same structure as the two previous chapters. It starts by evidencing how ‘positivist’ construed that international legal order. It shows how they kept but ‘positivized’ many naturalist premises, such as the principle of independence, equality and self-preservation. When it came to custom identification and determination, ‘positivist’ firmly advocated a ‘historical’ method. Consequently, when analysing the rules of intervention, positivist authors engaged in a much more thorough analysis of State practice than their colleagues. Notwithstanding, this still led them to conclude that non-intervention was the rule, and that instances of uses of armed force where the application of exceptions to this rule. Thus, even nineteenth-century ‘positivists’, who we today tend to portray as ‘neo-Hegelians’, believed the use of force was not an unrestricted prerogative of States.

This chapter is the first of three which attempts to question the assertion according to which the justifications brought forth by States when they resort to force were political and moral and had not legal content or reverberations. It focuses on instance of intervention in the 'centre', i.e., between so-called civilized nations. Five precedents are more particularly analysed in this Chapter: the Austrian intervention in Naples (1821), the Crimean War (1853–1856), the Franco-Prussian War (1870–1871), the Spanish-American War (1898), and the Austrian ultimatum to Serbia (July 1914). It shows how, in each of these instances, States took care to develop legal arguments to explain and justify their actions, sometimes even engaging in thorough debates about the legality of their respective behaviours.

Grotius recast Aristotelian theories of human sociability in terms of self-preservation. Religious war in Europe had undermined the Thomist notion of mutual human affection as a basis for society. If society was established by the need to survive, then justice, which maintained society, must be understood in terms of its contribution to that necessity. Grotius therefore resolved the Ancient Roman and Greek problem of how to reconcile justice and expedience by reinterpreting justice in terms of expedience. For an individual, or state, to act out of self-preservation was necessarily just. His fusion of justice and expedience was one reason he was insistent upon distancing his thought from the Academic Sceptics, such as Carneades, who argued that there was no such thing as justice and that all moral action was expedient. For Grotius, part of the law of self-preservation was the necessity for individuals to secure the means for self-preservation and this meant that the acquisition of property, and trade, were central parts of that process. These principles applied also to the artificial person of the state which found itself in competition for survival with other states. The expansion of the state was therefore justifiable for its preservation. Indeed, following this reasoning, empire effectively became a necessity, and an inevitability, for the survival of European states.

Hugo Grotius is considered one of the paradigmatic figures in international relations theory. His thought is often contrasted with that of Thomas Hobbes, who is portrayed as the standard bearer of political realism, and the universalist orientation of Immanuel Kant. The centre piece of the so-called Grotian tradition is the theory of international society, which accommodates the claims of independent states without granting them absolute justification. The pursuit of advantage is subject to common standards that oblige rather than merely counsel moderation and restraint. The chapter proceeds in three parts. Part one examines the legal and political narratives that account for the emergence of the Grotian tradition. Part two examines revisionist scholarship that considers Grotius’ thought in the context of relations between Europeans and non-European ‘others’. Neither the standard nor the revisionist narrative provides an adequate account of obligation, without which the theory of international society collapses in confusion. Part three responds to this problem by exploring a part of Grotius’ thought that has been excised from international relations theory: theology. This illuminates an account of obligation that rescues the Grotian tradition from the coarse world of moral scepticism and power politics.

Chapter 8 deals with exclusion. The inevitable conflict thesis errs when it asserts that all positive IHL rules, including those that are unqualified, involve military necessity and humanity in their norm-creation. This chapter shows, rather, that IHL norm-creation does not always involve the military necessity–humanity interplay. Ill-advisedly, the inevitable conflict thesis also holds that neither military necessity nor humanity may be invoked in defence of deviations from unqualified IHL rules. While Kriegsräson and its variations are properly excluded, humanitarian imperatives may justify non-compliance with IHL and/or render technical compliance unlawful.

On the Citizen is both the second version of Hobbes’s political theory, a precursor of Leviathan, and part of a scientific project titled “The Elements of Philosophy.” The chapter disentangles how the work relates to these different contexts. It examines the development of Hobbes’s political thinking by comparing On the Citizen with an earlier text (The Elements of Law), giving particular attention to his discussions of the state of nature, the political covenant, and governmental accountability. Turning to the second context, the chapter tracks the evolution of Hobbes’s thinking away from purely formal, definitional science. On the Citizen lays out the problem of the contestability of foundational concepts in moral and political philosophy and tries out a solution in the form of a concept of “right reason.” Alongside that ultimately unsatisfactory line of thinking, Hobbes also advances an empirical approach to science of a more political nature. As indicated by the title, On the Citizen was framed to cover political subjects, an empirical focus that propelled Hobbes’s development as a political theorist. Leviathan is a master work in political theory because it was preceded by On the Citizen.