1. Sources of International Law
Customary International Law

2. Sources of International Law Custom
Customary international law is the result of consistent practice (diuturnitas) of a majority of states supported by the belief that the practice is obligatory (opinio juris). Example: you cannot use your territory in such a way as to cause harm to other states; the territorial sea is 12 mile wide.

3. Sources of International Law Custom
Traditionally, most of international law rules were rules of customary international law. After WWII much of customary international law has been codified in treaties. Treaties are nowadays the major source of international law.
However, CIL still plays an important role because…

4. Sources of International Law Custom
There might be no applicable treaties. I.e. the issue is not regulated by a treaty or some or all of the parties involved in the issues are not party to the relevant treaty/ies.
CIL is the background, the fallback when treaties cannot be resorted to in order to determine what international law requires.

5. Sources of International Law Custom
Treaties bind only states that ratify them
Customary international law is law for all states, regardless of whether they participated in the formation of the custom. A state is bound by customary international law simply by virtue of being a state. Exception: persistent objector
A persistent objector is a state that has repeatedly and unambiguously objected to the emergence of a norm of CIL.

6. Sources of International Law Custom
Third Restatement of the Foreign Relations Law of the United States
“… custom is law that results from a general and consistent practice of states followed by them from a sense of legal obligation”
Two constitutive elements of CIL
Opinio Juris
Diuturnitas

7. Sources of International Law Custom
Both elements need to be present for a norm of CIL to exist.
Opinio juris without practice: A logical mistake.
Practice without opinio juris: mere courtesy, comity, good will. E.g. Saluting the flag of a foreign country. Providing assistance during natural catastrophes.

8. Sources of International Law Custom
Determining what CIL is an art, not science…..!!!
Opinio juris: What states say and do is important, but also whet they do not say and do not do. Silence = acquiescence. How do you know what’s states’ opinio juris? You look at how the behave…practice.

9. Sources of International Law Custom
Practice:
Whose practice? How many states? Forty? Fifty? More than half of all states? Two-thirds? Most? Equally distributed around the globe? Is it a matter of numbers or also of which states? If 180 states claim a right to have a territorial sea extending 200 miles from the shore (currently, the custom is twelve miles), but the United States, Russia, China, the United Kingdom, Brazil, Japan, Australia, France, Nigeria, Argentina, Chile, South Africa, Canada and India object and insist on twelve miles, is it still plausible to claim that 200 miles is the custom?
For how long does the behavior need to be practiced exactly? One day, one week, one month, one year, a decade, a century?
There is an inverse relationship between time and number of states participating to a practice. The more states, the less time is needed for a norm of CIL to emerge. The less states, the more time is needed.

10. Sources of International Law Custom
Where do you look to know what states’ practice is? You look at what states do and say. Any act that is attributable to the government is good. Treaty making, unilateral declarations, decisions within international organizations, national laws, military and police manuals, internal governmental circulars, press releases and conferences…..

11. Sources of International Law Custom
The paradox of CIL: to change as norm of CIL you need to violate it.
Jus cogens is a particular category of CIL that cannot be derogated by way of treaty. Even jus cogens could change overtime is states practice is sufficiently widespread and consistent.

12. Sources of International Law Custom
When is it necessary to determine what CIL requires?
Debate on what CIL requires is part and parcel of everyday interactions between states: e.g. debate on nationalizations and amount of compensation due; military use of outer space.
Judges (domestic and international) might need to determine what CIL requires to settle cases brought before them.
International: E.g. International Court of Justice (ICJ) on maritime delimitation, asylum, diplomatic immunities, etc.
Domestic: E.g. US SC in the Paquete Habana case

13. CIL in the US legal order
Customary International Law
“International law is part of our law, and must be ascertained and administered by the courts of justice of appropriate jurisdiction as often as questions of right depending upon it are duly presented for their determination. For this purpose, where there is no treaty and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages of civilized nations…”. Paquete Habana, 175 U.S. 677, 700 (1900).

14. CIL in the US legal order
General consideration (law and power):
Who controls the formation of CIL? On the one hand it is the tool of the poor/weak, who, by acting together consistently over time, might be able to impose their will on few powerful states. E.g. attempt to rewrite in international economic order.
On the other hand, because only a minority of states has the capacity to consistently act and being heard on their international scene, their practice tend s to carry greater weight that than of the many. E.g. the US rewriting the laws of war because it is the country that engages the most in military activities.