If mayors ruled the world” politics would be more pragmatic, solution-oriented and less polarised, the late Benjamin Barber argued in his famous book of the same title (Barber, 2013). Some of his assumptions were daring. Cities are hardly without issues such as lack of accountability or inequality, they depend on national decisionmaking in crucial policy areas such as migration, trade, and financing, and they have natural capacity limitations when it comes to foreign policy. Yet the days when international relations were the exclusive remit of high politics are over. Municipal and metropolitan actors play an increasing role, whether it is climate change mitigation, Track II diplomacy, educational initiatives, or the accommodation and integration of refugees and migrants.