IT WOULD BE DIFFICULT to overstate the ambition of “Postwar: Art Between the Pacific and the Atlantic, 1945–1965.” The exhibition includes hundreds of works by artists from six continents, the checklist is 192 pages long, and the catalogue weighs ten pounds. Yet the show’s material sprawl is dwarfed by an even more monumental historical-theoretical project. An exhibition summary provided by Haus der Kunst offers this description of the enterprise: “Probing differing concepts of artistic modernity . . . the exhibition explores how individual receptions and formulations of modernism informed the