The argument of this paper is based on a number of assumptions of which the following two are most crucial. In the first place, treaties are assumed to be a sub-species of promise. A treaty is thus regarded as a promise made between States, not distinguishable logically from promises made between individuals, or groups of individuals other than States. Secondly, promise-making is regarded as an institution, constituted by a conventional principle that the use of certain words, such as, " I hereby promise to do x," creates an obligation, within that institution, for the speaker to do x. Thus, in the following discussion the obligation to do x within that institution is treated as distinguishable from a less well-defined obligation of similar type: e.g., the obligation not to betray the expectation which one may be said to have induced in the minds of the hearers when one has unilaterally declared one's intention to do x, or when one has hinted at the possibility that one might do x in future.