Between 1930 and 1940, around 15 Swiss citizens lived in what at the time was the French Congo. Most of them were working as engineers, building the railway line between Brazzaville and Pointe-Noire.
Switzerland recognised the independence of the Republic of the Congo in 1960. Two years later, the two countries signed bilateral agreements on trade, investment protection and technical cooperation, followed in 1964 by an agreement on air traffic. Various agreements on debt relief and rescheduling were concluded in 1986, 1991 and 2005.
Historical Dictionary of Switzerland (de, fr, it)
Diplomatic documents of Switzerland, Dodis