  In spring 1949 about 70 physicists from eight countries met in Florence to
discuss recent trends in statistical mechanics. This scientific gathering,
co-organized by the Commission on Thermodynamics and Statistical Mechanics of
the International Union of Pure and Applied Physics (IUPAP) and the Italian
Physical Society (SIF), initiated a tradition of IUPAP-sponsored international
conferences on statistical mechanics that lasts to this day. In 1977, when this
conference series took the name of StatPhys, the foundational role of the
Florence conference was recognized by retrospectively naming it StatPhys1. This
paper examines the dual scientific and social significance of the conference,
situating it in the broader contexts of the post-World War II reconstruction in
Italian physics and of the revitalization of the international science
organization. Through an analysis of IUPAP archives and Italian records, we
illustrate how the event's success hinged on the aligned objectives of its
organizers. Internationally, it was instrumental in defining the scientific and
organizational foundations for the activities of IUPAP commissions during a
critical phase of IUPAP's history, when the Union was resurging on the
international scene after the inactivity of the interwar period. Nationally,
the conference served as a cornerstone in SIF's strategy to re-establish
Italian physics' international stature and to aid the domestic revitalization
of physics through the internationalization of its activities, notably of its
flagship journal, \textit{Il Nuovo Cimento}. This analysis not only sheds light
on the conference's impact but also informs recent discussions in the history
of science about the multiple roles of international scientific conferences.
