An oft-cited source of IHL when identifying its underpinning principles is the 'Martens Clause'. This was introduced for the first time in the Preamble to the 1899 Hague Convention II and has since acquired customary international law status. The Martens Clause states that even in situations not covered expressly by codified IHL instruments, both combatants and civilians have a minimum level of protection, namely that all hostilities should be regulated by the principles of the law of nations as they result from the usages of international law, from the laws of humanity, and from the dictates of public conscience. This reflects the overarching goal of IHL which is to establish minimum, non-derogable standards of restraint which apply in all situations of armed conflict.
The core fundamental principles of IHL are:
IHL is both simple and complex in terms of its objectives, underpinning principles and related challenges:
To put things as simply as possible, these rules can be summed up in four precepts: do not attack non-combatants, attack combatants only by legal means, treat persons in your power humanely, and protect the victims ... At the same time, the law of armed conflicts is complex since it does apply only in certain situations, those situations are not always easily definable in concrete terms and, depending on the situation, one and the same act can be lawful or unlawful, not merely unlawful but a criminal offence, or neither lawful nor unlawful! ... (David, 2002, pp. 921-922).
There are certain aspects that IHL does not regulate. For example, it does not prohibit the use of violence per se, nor is it concerned with the purpose of any conflict, nor does it protect all persons affected by armed conflict, especially combatants directly engaged in hostilities who may lawfully be killed.