Those Romans, always building arches.

The Arch of Augustus at Susa, the best preserved of all Augustan arches in italy.

On the road leading to the Alpine crossing to Gaul, in 9 BC Augustus erected this arch to celebrate the peace treaty he and Marcus Iulius Cottius had signed. A thin frieze on this arch depicts the treaty signing and the related ceremonies that accompanied it (close up here).

Cottius was the successor and son of the king of 14 tribes in the so-called Cottian Alps. Within the terms of this treaty was for Cottius to become a Roman citizen with the name Iulius, to renounce his kingship, and to become a local magistrate over the people he once ruled.

The Arch of Augustus celebrates the bloodless establishment of the Pax Augusta in this part of Italy. Augustus will boast about this in Res Gestae (5.26):

"The provinces of the Gauls, the Spains, and Germany, bounded by the ocean from Gades to the mouth of the Elbe, I reduced to a state of peace. The Alps, from the region which lies nearest to the Adriatic as far as the Tuscan Sea, I brought to a state of peace without waging on any tribe an unjust war." (trans. Shipley)