In the Nairobi Declaration of June 1963 the political heads of the three East African mainland states publicly committed themselves to federation; in June 1965 they were beginning to prepare the public for a substantial reduction in the area of functional co-operation that had been established prior to 1963. With the wisdom of hindsight we can perhaps see that the reasons for this development are fairly obvious. This paper chronicles some of them. Although the East African states had so much in common they had some very important differences. The optimists pointed to what they had in common and the pessimists to the differences. Optimists noted: (1) strong historical and cultural links, deriving from common history slavery and Swahili and British administrative institutions and education which were particularly important in elite circles; (2) far-reaching functional links common market, common services, free personal movement (more or less), common currency, etc.; (3) common economic and political interests the advantages of a large East African market for industrialization, diplomatic influence, etc. The pessimists noted: (1) differences in the character of the political systems of the three states, especially the constraints imposed on Uganda by Buganda's longstanding hostility towards East African union; (2) the tensions caused by conflict of national economic interests in operating the common market. It is possible to see very clearly now that the optimists, who argued that Wheare's classic statement of the prerequisites of federation was fulfilled and that it should therefore be possible to establish it without further ado, were victims of some large