Every nation is expected to obey international law. Some nations make international law automatically part of the law of their land. The U.S. Constitution designates ratified treaties, along with the Constitution itself and federal statutes, the supreme law of the land (Article VI) and empowers Congress “to define and punish … Offences against the Law of Nations” (Article I, Section 8). Customary international law is automatically incorporated into the U.S. legal system as federal common or unwritten law.
In cases involving international law, U.S. state and federal courts presume that U.S. law conforms to international law; such an attitude has been urged consistently by the Supreme Court of the United States. In some countries, such as the United Kingdom, treaties do not become effective in national law until they are enacted by Parliament. In other countries, a treaty or customary international law is given constitutional status superior to national legislation. How a sovereign state adopts and applies international law is generally left to its discretion, so long as it conforms to the law in the end.
Whatever the constitution or legal system of a nation, it cannot use its domestic law as an excuse to breach an international agreement or violate an international rule. This was made clear during the war crimes trials held in Nürnberg, Germany, following World War II. The Nürnberg tribunals rejected the defense that certain acts, such as the killing of prisoners of war, were permitted under the domestic laws of Nazi Germany. The tribunals held that such laws were null and void because they contravened the generally valid rules of warfare. It also held that the individuals responsible for issuing and executing such laws were criminally responsible for grave breaches of international law. Today, international human rights courts often declare national laws incompatible with international rules and may award compensation to those whose rights have been violated.