How and when states can use force legitimately is one of the most persistent and intractable problems of international relations. In part, this persistence and intractability stem from the rise and fall of very different ideas of legitimate force across the evolution of the international system. While ideas about the limits of force have changed, states have routinely justified their decisions, and invoked varieties of legal, ethical and constitutional principles in the process. Over time, these arguments have become more determinate and harnessed by actors on all sides for very different purposes. This has led to divergent and often competing visions of legitimate conduct. This dynamic and the confusion it creates are under -theorised in the International Relations discipline, partly because few texts work at dual levels of philosophical abstraction and historical breadth. In this space, my thesis proposes the existence of a unique relationship between legal, ethical and constitutional arguments about force. It offers a way to conceptualise how these arguments have evolved over time, and why they vary under hierarchical versus anarchical international orders.
To demonstrate my claims, I adopt a macro-historical approach, in which I select what seem to be the most defining cases in the relationship between legal, ethical and constitutional arguments about the legitimate use of force. They are times when the use of force prompted a significant level of debate about the basis of its legitimacy, and are structured along two axes of comparison. The first axis addresses cases both before and after the rise of legal positivism. This situates my study to include law’s religious foundations in the early modern period, which then shifted to a more scientific practical form during the nineteenth century. The second axis maps the terrain of intersubjective ideas about the use of force. This draws out the historical dimensions of ethical and constitutional arguments to consider the use of force both within and on the margins of international society. Four cases are included in the analysis. The Spanish conquest of the Americas occurred
prior to the rise of positive law. The use of force was justified as an effort to bring Christianity to the inhabitants of the New World, as vassals of the Spanish Empire. The Thirty Years War and the Peace of Westphalia fit the same pre-positivist period, but saw different groups within European Christianity use force to negotiate the differences of their broader faith. The next conflicts after the rise of legal positivism are World War I and World War II (1914–1945). These wars are examined
as a single integrated crisis, unfolding within the confines of a broader European/Western community that struggled to negotiate the vagaries of competing ideologies. The most contemporary case, the 1991 Gulf War and 2003 Iraq War, completes the project’s symmetry. The Gulf War began as a conflict between sovereign states, before escalating in 2003 as a war between the Western democratic community and a dictatorial ‘other.’