International law is a collection of complex and developing rules that governs the relationship between nations. The range of subjects directly concerned with international law has widened considerably, moving beyond the issues of war, peace, and diplomacy to include human rights, trade issues, space law, and international organizations.
Article 38 (1) of the International Court of Justice identifies three sources of international law: treaties, customary international law, and general principles of international law (jus cogens)
A treaty is the most preferred source of international law. It is an agreement entered into by sovereign states and international organizations. Treaties can be loosely compared to contract. Both are means of willing parties assuming obligations among themselves. If a party fails to live up to their obligations can be held liable under international law for that breach. The central principle of treaty law is expressed in the maxim pacta sunt servanda—”pacts must be respected”. The principle refers to contracts, stressing that contained clauses are law between the parties, and implies that non-fulfillment of respective obligations is a breach of the pact.
The Constitution of the U S stipulates that treaties shall be the supreme law of the land. Treaties are negotiated by the president but can be ratified only with the approval of two thirds of the Senate except in the case of executive agreements. Executive agreements are made by the president on his own authority. Further, a treaty may be either self-executing or non-self-executing, depending upon whether domestic legislation must be enacted in order for the treaty to enter into force. In the U S, self-executing treaties apply directly as part of the supreme law of the land without the need for further action. Whether a treaty is deemed to be self-executing depends upon the intention of the signatories and the interpretation of the courts.