MacInnes's Venetian Affair (1967).

Robert Vaughn (still playing Napoleon Solo on Man from U.N.C.L.E. at the time) tries to figure out why a US diplomat detonated a bomb at a peace conference in this adaptation of Helen MacInnes's spy novel (Kirkus review here). The film costars Boris Karloff, Elke Sommer, and Felicia Farr.