I - General Background

1.3.2 The Law of War’s Relationship to Other Bodies of Law.
An issue that often confronts law of war practitioners is the relationship of the law of war to other bodies of law, especially when rules in those bodies of law may appear to conflict with rules reflected in the law of war. These apparent conflicts are often resolved by considering the principle that the law of war is the lex specialis governing armed conflict.

The law of war has been developed with special consideration of the circumstances of war and the challenges inherent in its regulation by law. Thus, for example, the exigencies of armed conflict cannot justify violating the law of war. Moreover, lawmakers sometimes have considered peacetime rules appropriate to apply during armed conflict, and in certain of these cases, they have explicitly incorporated such concepts into the law of war.

Thus, traditionally, the law of war has been described as the only “authoritative rules of action between hostile armies,” or as superseding ordinary law in the actual theater of military operations. Similarly, law of war treaties have been viewed as a clear example of a lex specialis in relation to treaties providing peacetime norms concerning the same subjects.

1.3.2.2 Construing Other Laws to Avoid Conflict With the Law of War.
Potential conflicts between the law of war and other law may be resolved by construing such other law to avoid conflict with law of war rules. Underlying this approach is the fact that the law of war is firmly established in customary international law as a well-developed body of law that is separate from the principles of law generally applicable in peace.

1.4.2.2 Nature of War – Limited and Unreliable Information – “Fog of War”.
During war, information is often limited and unreliable. The uncertainty of information in war results from the chaotic nature of combat and from the opposing sides’ efforts to deceive one another, which generally is not prohibited by the law of war. The limited and unreliable nature of information available during war has influenced the development of the law of war. For example, it affects how the principle of military necessity is applied. The limited and unreliable nature of information available during war also is recognized in the law of war’s standards for how persons are to assess information.

1.5.2 Different Definitions of “War” for Different Legal Purposes.
There is no single legal definition of “war,” “hostilities,” or “armed conflict,” and the definition of these terms has varied in both domestic and international law.

In domestic law, “war,” “hostilities,” and “armed conflict” have been interpreted differently depending on the specific legal context at issue. For example, under the Constitution, Congress has the power to “declare war.” Thus, “war” might be interpreted to determine whether a military operation constitutes “war” in this sense. Similarly, the War Powers Resolution states certain requirements that are triggered when U.S. forces are introduced into “hostilities.” Other statutes may require a determination that conduct has occurred “[w]hen the United States is at war” or during “time of war.” Under international law, “war,” “hostilities,” and “armed conflict” may also be interpreted with different purposes in mind. A state of “war” can affect what duties States that are not participating in the conflict have under the law of neutrality. A state of “war” can affect whether peacetime treaties between two States continue to apply. Most importantly for the purposes of this manual, the terms “war” and “armed conflict” are used to describe when jus in bello rules apply.

1.11.4 Rationales for the Resort to Force.
1.11.4.1 Use of Force in Self-Defense.
The right to use force in self-defense is an inherent right of States.

1.11.4.2 Use of Force Authorized by the U.N. Security Council Acting Under Chapter VII of the Charter of the United Nations.
Chapter VII of the Charter of the United Nations provides that the U.N. Security Council may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace or security, including demonstrations, blockade, and other military operations.

1.11.4.3 Use of Force With the Consent of the Territorial State.
Military action in the territory of another State is not a violation of Article 2(4)’s prohibition against the use of force against that State where it consents to such military action.