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 ‘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 stillled 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.