Customary Law and International Treaties Application
Application of Customary Law
First problem: To demonstrate the existence of an unwritten rule, you must prove its two constituent elements:
- A constant and uniform practice.
- The conviction that this practice has become compulsory (opinio iuris).
Second problem: Proving that this custom applies to the other party. Evidence can be found in:
- Diplomatic correspondence between states, which can reveal positions on particular situations.
- Conventional practice of states, deduced from international treaties (e.g., reservations or acceptance by other states).
- Government statements on issues relating to international relations.
- Projects and positions of state representatives at international conferences, organizations, or courts.
- Laws.
- Case law.
- Decisions of the government in international relations.
- Parliamentary projects.
- Case problem solvers regarding immunity from jurisdiction or execution.
- Decisions of international courts, indicating the presence or absence of an international custom.
- Resolutions of international organizations, especially those of the UN General Assembly. A unanimous or overwhelming decision confirms opinio iuris.
- Work of international bodies, particularly the International Law Commission.
Application of Conventional Norms
- Proving the existence of the treaty at the time of application is crucial.
- International treaties prevail over domestic rules. The Vienna Convention of 1969 states that provisions of domestic law cannot justify non-compliance with international standards.
- International treaties do not produce legal effects for third parties without their consent.
- International treaties are applied from their entry into force until their termination, or from provisional application until declared void, suspended, or their duration ends.
Application of Successive Treaties on the Same Subject
When two successive treaties on the same subject are in effect, and the second is not an amendment of the first, the following rules determine which treaty applies:
- If any of the treaties is the UN Charter, it takes precedence.
- One treaty may resolve conflicts with another by being considered complementary or supplementary. In that case, the later treaty applies over the earlier one’s provisions that are not inconsistent.
- When some of the states in the first treaty are the same as those in the second, the rules of the later treaty, as well as the earlier ones not incompatible with it, apply.
- If not all states are part of both the first and second treaties, apply these rules:
- Relations between states party to both: The second treaty applies, and the earlier ones that are not incompatible.
- Relations between a state party to both and one party to only one: The treaty to which both are party applies.
- Relations between states party to only one treaty: The treaty in which they are both part applies.
Application and Interpretation by State Organs
Application: State bodies implement international standards, sometimes requiring an express act of domestic reception, and sometimes not.
- In the case of international custom, if it compels Spain, it may be applied without an act of reception. Courts must consider that they may apply rules containing provisions for individuals. All others will be implemented by the Ministry of Foreign Affairs (AAEE).
- For conventional rules, Spain must have expressed consent to be bound by the treaty, which must be in force. The international standard should be part of domestic law (achieved by publishing in the Official Gazette). If not, courts may not apply it.
- Another condition for applying conventional rules is conformity with the European Community (EC). A judge with doubts should raise a constitutionality issue.
- Not all customary rules are self-executing. Some need internal rules for development (94.1 EC). If so, the State Council should be consulted. Self-executing norms are directly applicable once published in the BOE.