When it comes to outsized projects with geopolitical undertones, Walter Smerling has form. Between 2015 and 2017, the chairman of the Stiftung für Kunst und Kultur (Foundation for Art and Culture) in Bonn and a German art-world statesman of sorts organised the largest ever exhibitions of Chinese art in Germany and German art in China. And when tensions between western Europe and Russia started rising around the same time, he envisaged a continent-spanning exhibition: “A dialogue,” as he puts it, “about what unites people in Europe. About democracy, about solidarity, about personal and political freedom.” The result is Diversity United, a traveling group exhibition comprising over 150 artworks by around…