Reservations are, perennially and by acclamation, one of the most complex and controversial parts of treaty law. The idea is simple-states may ask that treaty terms be tailored to their individual preferences, turning a prix fixe menu a la carte-and critical to establishing and applying international obligations. But the same issues have bedeviled the law for over fifty years, notwithstanding repeated attempts to resolve them. Shortly after the United Nations was founded, the General Assembly sought and obtained advice on reservations from the International Court of Justice and the International Law Commission, then adopted a resolution characterized as "one of the fundamental documents in the history of the law of treaties." Fifteen years of further debate produced an important section of the Vienna Convention on the Law of Treaties that governs, by default, how reservations are handled. Basically, states are not supposed to propose reservations that are incompatible with a treaty's "object and purpose." But if a state proposes a reservation, and another state does not object within a year, the reservation is said to modify the treaty as between them.
Edward T. Swaine,
Yale J. Int'l L.
Available at: https://digitalcommons.law.yale.edu/yjil/vol31/iss2/2