As a reaction to the changed political landscape in Scandinavia following the dissolution of the union between Norway and Sweden in 1905, the prominent Swedish mathematician Gösta Mittag-Leffler extended ‘a brotherly hand,’ calling for Scandinavian colleagues to meet for a congress of mathematicians in Stockholm in 1909. This event became the first in a series of biannual meetings which proved to be an important institution for Scandinavian mathematics. During the first decades after 1909, the congresses would form and consolidate themselves through the construction of a new Scandinavian identity for mathematicians which developed alongside and in relation to both international and national contexts and developments. In this paper, we shall demonstrate that these meetings served a complex set of agendas at the individual, national, and international level. In particular, they reflect a changing conception of cooperation in science for mutual cultural gain combined with a flexible institutionalisation that allowed the Scandinavian mathematicians to use the congresses for various diplomatic ambitions. We base our analyses of the Scandinavian Congresses of Mathematics on the notion of a shared ‘conational’ identity developed adjacent to national identities. We then analyse the formation, consolidation, delineation, and reflections of this institution in order to demonstrate how the efforts to unite Scandinavian mathematicians were contingent on and influenced by simultaneous currents of internationalisation and shared history, culture, and language in the Scandinavian region.