The Arch of Galerius

 The Roman emperor Galerius commissioned the triple Arch of Galerius, built in 298-299 CE and dedicated in 303 CE,  as an element of the imperial precinct linked to his palace in Thessaloniki, Greece. Galerius served as Caesar during the tetrarchy of Diocletian and married Diocletian's daughter.  Entrusted with the care of the Illyrian provinces, he campaigned against Sarmatians and Goths along the Danube then was  dispatched to Egypt to fight the rebellious cities Busiris and Coptos. From there, Galerius was sent to command the eastern forces between Carrhae and Callinicum in Syria.

Then, in about 295 CE, Narseh, son of the king Shapur I and the seventh emperor of the Sassanid Persian Empire, declared war on Rome and invaded western Armenia. Narseh then moved south into Roman Mesopotamia, where he inflicted a severe defeat on Galerius. Diocletian may or may not have been present at the battle, but presented himself soon afterwards at Antioch, where the official version of events was made clear: Galerius was to take all the blame for the affair. In Antioch, Diocletian forced Galerius to walk a mile in advance of his imperial cart while still clad in the purple robes of an emperor. The message conveyed was clear: the loss at Carrhae was not due to the failings of the empire's soldiers, but due to the failings of their commander. 

However, after Galerius' army was reinforced in 298 CE, Galerius lead an offensive into northern Mesopotamia by way of Armenia. To Narseh's disadvantage, the rugged Armenian terrain was favorable to Roman infantry, but not to Sassanid cavalry. Local aid gave Galerius the advantage of surprise over the Persian forces, and, in two successive battles, Galerius secured victories over Narseh. At the Battle of Satala, Roman forces seized Narseh's camp, his treasury, his harem, and his wife. Narseh's wife would live out the remainder of the war in Daphne, a suburb of Antioch, serving as a constant reminder to the Persians of the Roman victory. The Romans, in any case, treated Narseh's captured family well, perhaps seeking to evoke comparisons to Alexander and his beneficent conduct towards the family of Darius III.

At the crux of the major axes of the city of Thessaloniki, the Arch of Galerius emphasized the power of the emperor as embodied by his conquests.The arch is composed of a masonry core faced with marble sculptural panels celebrating his victories. 

I found this closeup of one of the arch's panels up on Flickr courtesy of Dan Lundberg  (cc by sa 2.0).  In the center an eagle brings a victory wreath to a mounted Galerius who is overpowering the Persian shah Narseh on his own horse—a confrontation that never occurred.

More images of the arch panels can be found here:

https://commons.wikimedia.org/wiki/Category:Arch_of_Galerius_(Thessaloniki)_-_Reliefs