This is precisely what the Americans (but also France) fear: that the Chinese will eventually force the Djibouti government to take decisions contrary to American (and/or French) interests. For example, the US base in Djibouti - which is the largest US base in Africa - depends on the container terminal at the port of Doraleh for its supply. Until February 2018, it was controlled by an Emirati company, but the government nationalized it, alienating the UAE in the process (which only strengthened their determination to work for Ethio-Eritrean peace to give this landlocked giant Ethiopia an alternative to Djibouti for its access to the sea). The fear now is that the Djibouti government will entrust the management of the terminal to China, which will then be in a position to affect the supply not only of the American base but also of US Navy vessels in the region.
This development has de facto eroded the United States' confidence in Djibouti and, as in the case of the UAE, has contributed to strengthening its motivation for Ethio-Eritrean peace, which has the advantage of reducing Djibouti's monopolistic situation and therefore the magnitude of the risks involved.
In addition to French military interests in Djibouti, what is the nature of the interests of the region that President Macron defended during his visit?
On a continental scale, there is, first of all, an interest in terms of the perception of France's "African policy" which, for legitimate reasons, is very present in the West, and which must therefore regularly show that it does not forget the East. In Nairobi, President Macron recalled that "as crazy as it may seem, this is the first visit of a French president to Kenya". This rebalancing is all the more necessary as the region has many assets and is the link of a "great game" between the great powers. We have talked about the United States and China, but Russia could be discussed too, particularly in Sudan.
Indeed, there are military interests in Djibouti, with the largest French contingent in Africa, many of whom have come with their families, which is also significant for the French presence in the country. This presence is not only a colonial heritage, it now has a triple legitimacy: the protection of French nationals and maritime traffic along one of the world's main trade routes; the protection of an area of responsibility formalised by defence agreements with not only Djibouti but also the Comoros; and finally, its overseas departments and regions in the Indian Ocean (Réunion and Mayotte) make France a neighboring country. Djibouti also offers excellent training opportunities, in terms of open areas and firing ranges, for the operational preparation of the military, in climatic and geographical conditions very close to those of current theatres of operations, particularly in the Sahel-Saharan strip (Operation Barkhane).
Given these strong and long-standing ties, President Macron could not make a regional tour without visiting Djibouti. In addition, the country needed to be reassured of France's support at a time when Paris' relative weight in the country is decreasing, while Beijing's is increasing, and Djibouti fears suffering from Ethio-Eritrean peace, i. e. competition from Eritrean ports - a fear which I believe must be put into perspective. Assab and/or Massawa do not have the qualities, in the medium term, to become serious alternatives to Djibouti. But this fear suits Addis Ababa in the meantime, because this psychological competition is putting pressure on Djibouti in terms of price and quality of service.