International humanitarian law

1. Ius ad bellum & ius in bello: Hague law and Geneva law
There is a distinction between ius ad bellum and ius in bello, the first regarding the waging of war, the latter regarding warfare. Armed conflict is governed by the Hague law on one hand, aiming to curb the conduct of hostilities, and Geneva law, aiming to protect the victims of an armed conflict.
Both the Hague law and Geneva law are recognized as a fundamental part of customary international law, as the Court states in para 79, 80 of the Nuclear Weapons Advisory Opinion (a reference to the 1949 Corfu Channel case is made):

"It is undoubtedly [..] rules of humanitarian law applicable in armed conflict are so fundamental to the respect of [..] "elementary considerations of humanity", that the Hague and Geneva Conventions are to be observed by all States, whether or not they have ratified the conventions that contain them, because they constitute intransgressible principles of international customary law" (para 79). Furhermore, the Court notes that these principles of humanitarian law are part of ius cogens (para 83).

From this follows that, regardless of the act of ratification by States, the rules codified in the Hague and Gevena Conventions apply to armed conflict, because these conventions embody the rules of customary international law.

1.2. The Hague law
Since the 1868 St. Petersburg Declaration, the Hague law has incorporated conventions to outlaw certain types of weaponry, as the 1993 Chemical Weapons Convention is one of these coventions being considered part of the Hague law.

1.3. Geneva Conventions and their Additional Protocols
The four Geneva Conventions are, in a particular order, as follows:
Convention I:    protection of wounded and sick members of armed forces;
Convention II:   protection of wounded, sick and shipwrecked members of armed forces at sea;
Convention III:  treatment of prisoners of war;
Convention IV:  protection of civilians

The three additional protocols are:
AP I:                detailed rules of protection of victims in international armed conflicts (IAC);
AP II:               regulation of non-international armed conflicts (NIAC);
AP III:              rules on additional distinctive emblem of the red crystal

2. The scope of humanitarian law: defining an armed conflict
There always uncertainty on the precise scope of humanitarian international law. Common Article 2 of the Geneva Convention provides that the Convention "shall apply to all cases of declared war or any other armed conflict which may arise between two or more of the High Contracting Parties, even if the state of war is not recognized by one of them".

Hence, the concept of an "armed conflict" has to be defined. The ICTY Appeals Chamber does so in the 1995 Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction in the Tadić case (2 October 1995): "..we find that an armed conflict exists whenever there is a resort to armed forces between States or protracted armed violence between governmental authorities and organized armed groups or between such groups within a State. Humanitarian law extends beyond the cessation of hostilities" (para 70).

As you might have noticed, the threshold for a NIAC is higher than is the case with an IAC. Humanitarian international law applies at the very moment of any act of force between States- with respect to international humanitarian law, one could say it is "war at first shot", as Common Article 2 of the Geneva Convention implicitly makes clear. NIACs, on the other side, require "protracted or large-scale armed violence" for international humanitarian law to be applicable.

2.1. Lotus doctrine and si omnes clause
The rationale behind this distinction is State sovereignty. International law would foremost be applied in conflicts between States, or High Contracting Parties to the Conventions. A si omnes clause has a very limited scope, stipulating that provions could only apply if States, party to a conflict, were both parties to the convention that would be applied. According the Lotus doctrine, only the specific prohibition of methods of warfare would govern interstate conflicts; otherwise, much was permissible.

2.2. Martens' clause
Фёдор Фёдорович Мартенс, Fjodor Martens, designed the general savings clause. This clause is included in Article 1(2) of AP I; furthermore, Common art. 2 of the Geneva Convention expressly rejects the si omnes clause. It does so by stating "Although one of the Powers [to a conflict] may not be a party to the present Convention.."
Note that paras 1-3 from Art. 60 of the VCLT do not apply to provisions relating to humanitarian treaties, Art. 60(5) VCLT. This provision stresses the importance of international humanitarian law: treaties concerning humanitarian law should not be terminated along the line of material breach by another party.

3. Principles of humanitarian law
Four principles underlie the rules of international humanitarian law as expressed in the conventions and additional protocols:
I.    Principle of distinction;
II.   Principle not to cause unnecessary suffering/ humanity principle;
III.  Principle of military necessity and proportionality;
IV.  Principle of precaution.

3.1. Principle of distinction
A distinction must be drawn between combatants ( defined by art. 43 of AP I) and civilians (defined by Art. 50 AP I).  A civilian is any person who does not belong to one of the categories referred to in Art. 4 (A)(1)(2)(3) and (6) of Convention III and art. 43 of AP I. Unless they take direct part in hostilities, civilians shall enjoy protection (Art. 51 (3) AP I). As Articles 48 (basic rule), 51(2) and 52(2) express, civilians cannot lawfully be made objects of attack. Note that enemies hors de combat will be safeguarded by Art. 41 of AP I; Article 41(2) AP I defines persons hors de combat which cannot be made objects of attack.

A combatant cannot be deprived of his right to be combatant, whether he has violated rules of international law or not, Art. 44(2) AP I. Members of the armed forces of a Party to a conflict, have the right to participate directly in hostilities, Art. 43(2) AP I. Any combatant as defined in Art. 43, who falls into the power of an adverse Party, shall enjoy the prisoner-of-war-status, Art 44(1) AP I. If a combatant fails to distinguish himself from civilians, Art. 44(3) AP I, he will not enjoy the prisoner-of-war-status, Art. 44(4) AP I; however, he will be given the equivalent of the protection accorded by Convention III and AP I. In order to garantuee as much protection as possible to all categories of combatants and non-combatants, main rule prescribes that anyone who cannot be considered a combantant in line with Art. 43 AP I, will be considered a civilian, Art. 50 AP I.

Civilians that take part in hostilities, become objects of attack (Art. 51(3) AP I), but they will not enjoy the status of prisoner-of-war when captured; if article 4 of Geneva Convention III does not apply, Article 45(3) in conjunction with Article 75 AP I affords the person, who is not entitled to prisoner-of-war-statues, the right to protection. Otherwise, Geneva Convention IV may apply.
There is a difference between civilians taking part in organized hostilities and civilians participating in levée en masse. The latter, spontaneously participating in non-organized hostilities, do become prisoners-of-war in the event of being captured, Article 4A (6) of Geneva Convention III.

Indiscriminate attacks are prohibited, Art. 51(4) and (5) AP I. Some conventions are designed to expressly prohibit the use of certain weaponry, such as the 2010 Convention on Cluster Munitions.

The Threat and use of nuclear weapons Advisory Opinion of 8 July 1996 discusses both the principles of distinction and the principle not to cause unnecessary suffering in paras 95-97. The Court's summary of the principles that constitute "the fabric of humanitarian law" is to be found in para 78.

3.2. Principle not to cause unnecessary suffering (humanity)
Necessity cannot be invoked to justify military actions at any cost, as Martens' clause aims to codify. The basic rules in Art. 35 of AP I stress this principle not to cause unnecessary suffering as a result of warfare. From the application of Common Article 3 follows that the humanity principle applies to NIACs as well.

Regarding Common Article 3, the ICJ notes in the Nicaragua case that:

"The importance of the principles that the Geneva Conventions and other relevant instruments purport to codify is [..] demonstrated by Articles 63, 62, 142 and 158 of Geneva Conventions I, II, III and IV respectively. These provisions allow for state parties to denounce the conventions unilaterally. However, the denunciation: "Shall in no way impair the obligations which the parties to the conflict shall remain bound to fulfil by virtue of the principle of the law of nations, as they result from the usages among civilized peoples, from the laws of humanity and the dictates of public conscience" (para 218).

3.3. Principle of military necessity and proportionality
As the Court states in the Threat or use of nuclear weapons Advisory Opinion, it is prohibited to use weapons causing the combatants unnecessesary harm, that is to say, a harm greater than unavoidable to achieve legitimate military objectives (para 78).

The last sentence of Art. 51(5)(b) AP I classifies "attacks that are excessive in relation to the military advantage anticipated" as indiscriminate and therefore prohibited.

To NIACs, customary international law imposes on parties to the conflict an equal obligation to refrain from causing greater harm than unavoidable to achieve military objectives.

3.4. Principle of precaution
The principles of humanity and military necessity imply the principle to take precautionary measures. In the conduct of military operations, Art. 57 AP I stipulates that constant care shall be taken to spare civilians and civilian objects. While AP I does not apply to NIACs, customary international law imposes on the parties to a NIAC the obligation to take precautions in order to prevent unnessary suffering.

4. NIACs
4.1. Defining NIACs and the application of Common Art. 3
On deciding whether the armed conflict in the Tadić case could be considered an international armed conflict, the Appeals Chamber in the Judgment of 15 July 1999 stated that "in order to attribute the acts of military or paramilitary groups to a State, it must be proved that the State wields overall control [..]" (para 131).

Although its scope is limited, Geneva Convention Common Art. 3 applies to NIACs. In the Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction of 2 October 1995 in the Tadić case, the ICTY Appeals Chamber makes clear that a higher threshold has to be passed for an internal conflict to qualify as a NIAC (para 70).

In the Tadić trial of 7 May 1999, the ICTY Trial Chamber has set out which requirements have to be met in order to pass the threshold for the application of rules regarding a NIAC: "The test applied to the existence of an armed conflict for the purpose of the rules contained in Common Article 3 focuses on two aspects of a conflict: the intensity of the conflict and the organization of the parties to the conflict" (para 562).

4.2. NIACs and AP II
Many of the provisions of AP II represent customary international law. The threshold in Additional Protocol II (which governs NIACs) can be derived from Article 1(1) AP II:

"..organized armed groups which, under responsible command, exercise such control over a part of its territory as to enable them to carry out sustained and concerted military operations".

The scope of AP II is, thus, limited, as Article 1(2) AP II puts it: "This Protocol shall not apply to situations of internal disturbances and tensions such as riots, isolated and sporadic acts of violence."

Like I noted before, the explanation for the restricted reading of the regulation of NIACs is State sovereignty. Also, States might not want to qualify internal conflicts with armed (rebel) groups as NIACs, for qualifying a conflict as such, implicitly means recognizing and eventually legitimizing the existence and actions of those groups.

5. The value of humanitarian law
What is exactly the value of international humanitarian law? Although the (codification of) customary international law aims to guarantee human rights and stems from universal human ideals of what global society should be like, it goes to show that many (non-) state actors don't live up to guarantee those rights. When non-State actors and States lack natural authority, one could image well how these actors resort to the threat with or use of violence and how they would rely on an arms race to scare off future, existent or imaginary enemies; living up to their human rights commitments would be the last to be concerned about. In practice, non-compliance is the rule rather than the exception. Even if all parties to a (human rights) treaty give their consent to be bound, there will always be ambiguity when conflicts arise. That is why many of the rules of international law apply in theory, but not in practice.

Recommended reading:
Typology of armed conflicts in international humanitarian law, S. Vité, March 2009;
International Humanitarian Law, H.P. Gasser, March 2011, available at Oxford PIL.