The triumphal arch holds a very important place in the annals of Western architecture and urbanism. In Roman times it played a lofty honorific role, even though most Roman arches commemorated civic achievements and personages rather than military victories. The freestanding monumental arch was a syncretic creation, structurally derived from Etruscan gateways and decoratively enriched with Hellenistic architectural and sculptural forms. Its distinctive presence, which accommodated an astonishing variety in massing and detail, enriched towns and cities across the Roman empire, from Spain to Syria. The monumental arch should be regarded as a universal entity—eminently appropriate, one would think, to...