The ABM Treaty is at least confusing and ambiguous on the core question of whether it applies only to traditional ABM systems. Although it is certainly correct to state that Agreed Statement D did not amend the treaty, it is equally clear that a treaty is to be interpreted in light of any agreement relating to it that is made by the parties in connection with the conclusion of the treaty. On the basis of this principal, and the established principle that all related parts of an agreement must be read together to give meaning and consistency to the whole, analysis based on the entire text and the public record militates in favor of a conclusion that the ABM Treaty prohibits deployment but not research, development, or testing of space-based BMD weapons based on new physical principles, such as SDI weapons.
Leon E. Irish,
SDI and the ABM Treaty: Problems of Negotiation and Interpretation,
Law Quadrangle (formerly Law Quad Notes)
Available at: https://repository.law.umich.edu/lqnotes/vol30/iss3/6