Athens raised the arch around 131 or 132 AD, almost certainly to mark the arrival of the emperor Hadrian and to thank him for his many gifts to the city. The timing was deliberate. The dedication coincided with the completion of the colossal Temple of Olympian Zeus just beyond, a project begun centuries earlier and finally finished under Hadrian's patronage. For a long time scholars believed the arch literally divided the old Greek city of Theseus from a new Roman quarter Hadrian had added on the eastern side. That tidy story has since collapsed. Excavation uncovered a gate of the older Themistoclean wall roughly 140 meters to the east, so the arch never stood on that boundary line at all. The inscriptions, it turns out, were making a claim about meaning rather than mapping a wall.

Hadrian was no ordinary conqueror passing through. He had become an Athenian citizen nearly two decades before this arch was built, and he loved the city with an intensity that shaped its skyline. That fondness complicates the second inscription. Read coldly, "the city of Hadrian, and not of Theseus" sounds breathtakingly arrogant for a foreign ruler to stamp on a Greek monument. So some scholars argue the arch was raised by the Athenians themselves, honoring Hadrian as one of their own rather than as Rome. Others credit the Panhellenes, a new league of Greek cities based in Athens. The same scale and design appear in two arches built a few decades later at the sanctuary of Demeter and Kore in Eleusis, also dedicated to an emperor by the Panhellenes. Whoever commissioned it, the message was generous, not servile: a refounder, claimed by the city he refounded.

The whole monument is solid Pentelic marble, quarried from Mount Pentelikon some 18 kilometers to the northeast, the same vein that built the Parthenon. The masons used no cement and no mortar. They cut the blocks precisely and locked them together with metal clamps, raising a structure 18 meters high and 13.5 meters wide that has stood almost entirely on the strength of stone fitted to stone. The lower level frames a single arched passage flanked by Corinthian columns. Above it rises a lighter, more delicate attic of columns and pilasters built around a central pediment, an upper storey so refined it could never have carried the heavy bronze chariots that crowned typical Roman triumphal arches. This is a Roman form rethought in a Greek key.

When the British architects Stuart and Revett made the first complete study of the arch in the 1750s, they found its base buried in only about three feet of earth. The arch had never been swallowed by the ground and protected, the way so many ancient monuments were, yet it survived in extraordinary condition anyway. The columns of the lower level are gone now, but the gateway still rises to its full height above the traffic of modern Amalias Avenue, named for Queen Amalia, who once ordered a stone screen removed from the upper niche for the sake of the view. Recent decades have been harder on the marble than the previous eighteen centuries were. Atmospheric pollution has discolored the stone and worn at the very inscriptions that make the arch worth arguing about. Stand beneath it at the right hour, with the Acropolis behind and the temple columns ahead, and you can still read both claims, and still feel the city refusing to settle the question.