International Law Domestic
Incorporation & International Responsibility
A State does not incur international responsibility simply by failing to bring national law into accordance with international obligations. International responsibility arises only when the State fails in a concrete case to fulfil international obligations. In some cases, the rights granted by domestic statute may be broader than those required by international law.
National Law in International Courts
National law may be presented before International Law Courts and Tribunals, showing that a State has accepted a particular state of affairs. National law may carry significant weight in showing whether certain practices and rules have been accepted as rules of customary law. However, it is a matter for the International Court to decide whether the national law is sufficient to fulfill the relevant international obligation.
International Tribunals generally settle disputes with reference to international law. However, it may be necessary to refer to concepts under national law. For example, certain international jurisdiction depends on whether a person is a national of a State. Nationality is defined by the national laws.
National law must be proven as a matter of fact before the International Court, supported by experts and lawyers in the relevant field. International Courts may determine the adequacy of the national rule in meeting international obligations. This, of course, does not have the effect of declaring the national law void unless the relevant jurisdiction has accepted the power of the Court to do so.
A national court may empowers the national legislation void for lack of compliance with international law. However, this matter exclusively lies within the jurisdiction concerned.
Incorporation into National Law I
The application of international law domestically raises the question of incorporation and transformation of international law into domestic law. In a monist jurisdiction, international law becomes part of national law without being adopted by the local legislature. Automatic adoption may apply unless there is some national law or decision to the contrary, implicitly excluding the same.
States may provide in their Constitution for the automatic incorporation of certain international rules. Common law countries may adopt certain international customary law to a certain extent without express incorporation. Transformation involves the enactment of international rules as part of national law, consistent with the dualist approach.
The distinction between incorporation and transformation does not necessarily coincide with the concepts of monism and dualism. In common law systems, international customary rules of law may be incorporated automatically into domestic law. The UK appears to adopt incorporation for customary international law and transformation for treaties.
Incorporation into National Law II
The Draft Declaration on the Rights and Duties of States 1949 provides that each State has a duty to carry out its obligations arising from treaties and other sources of international law in good faith. It may not invoke provisions of its constitutional laws as an excuse for failure.
The Vienna Convention on the Law of Treaties also provides that a party may not invoke the provisions of its internal law as a justification for failure to perform a treaty. This is without prejudice to Article 46, which provides that a State may  invoke a violation of a provision of its internal law if the violation was manifest and concerned a rule of its internal law of fundamental importance. Violation is manifest if it would be objectively evident to any State conducting itself in accordance with normal practice and good faith.
The common law has held since the 18th century that the law of nations is incorporated into the law of England. The UK position is that there is automatic incorporation unless there is a legislative provision or judicial decision of a higher court to the contrary. In the event of conflict, the domestic statute will be enforced.
Treaties made under the royal prerogative by the executive must be incorporated into domestic law by means of legislation. The royal prerogative may not alter law or confer rights on individuals or deprive them of rights as they enjoy under domestic law without the intervention of Parliament.
UK Practice
The Ponsonby Rule was a constitutional convention requiring that treaties be laid before Parliament 21 days before ratification and publication in the UK treaty series. The Constitutional Reform and Governance Act 2010 gives Parliament a role in the ratification of treaties. It puts the above conventional rule on legislative footing.
The government has a statutory duty to publish treaties subject to ratification or equivalent and lay it before Parliament for 21 sitting days. Since 1997, explanatory rules have been prepared and laid before Parliament for each treaty.
An executive certificate is a statement by the foreign office relating to categories of questions of fact in international affairs. It is accepted by courts as conclusive evidence, even if contrary evidence is forthcoming. However, executive certificates are not conclusive regarding the construction or interpretation of documents.
US Position
In the US, international law forms part of common law in accordance with similar principles as above. Customary international law is accepted in the absence of domestic decisions or legislation to the contrary. Statutes inconsistent with the principles of customary international law may lead to international law violations, but this does not have an effect domestically in the absence of incorporation.
The President has the power, following the advice and consent of the Senate, to make treaties provided two-thirds assent. Treaty law is supreme in the United States and has the same status as an Act of Congress.
Acts of Congress are interpreted as an order to conform with international law. Executive agreements, which may be entered by the President without involving the Senate, are potentially of the same force as a treaty.