As discussed in Chapter 1, there has always been tension between the desire to demonstrate humanity and the desire for security, which requires ‘continual adaptation following the needs of a changing world’.1 Attempts to address this tension have resulted in many treaties and agreements. Prior to written agreements, the behaviour of professional soldiers was, to a large extent, regulated by unwritten rules and customs.2 It has been suggested that ‘a large part of the modern law of war has developed simply as a codification and universalization of the customs and conventions of the vocational/professional soldiery’.3 However, these understandings were not codified, nor did States seek their codification, in an international form until the late nineteenth and early twentieth centuries: the Lieber Code in 1861,4 A Memory of Solferino (a text calling for codification of the principles of the laws of war in 1886),5 the Brussels Declaration in 1874,6 the Oxford Manual in 1880,7 the 1899 Hague Regulations (revised in 1907)8 and, more recently, the four Geneva Conventions9 and their three additional Protocols.10 These principles and rules, now codified, exemplify the behaviour that States expected from each other prior to treaties being negotiated that codified these expectations.