Wonder if he's thinking of the invasion of Mesopotamia in CE 165-166 in the reign of Marcus Aurelius. It was launched from the north (pretty much from the staging area denied to the US by the crucial Turkish parliament vote of March 1, 2003). By the end of 165 the war appeared virtually over, the Roman army had taken the twin cities of Seleucia and Ctesiphon (modern metropolitan Baghdad). In 166 the last resistance was suppressed in Media, and the triumph took place in Rome on October 12, 166.
But the returning army brought the plague with it.
"[The plague] had been caught first by the army in Mesopotamia, and the returning troops spread it all over the empire. 'It was his fate', his biographer says of Lucius [Verus - co-emperor with Marcus and supreme commander of the Roman army in the east], 'to bring the plague with him to those provinces through which he made his return journey, right up to Rome.' ... The effect and scale of this epidemic are a matter of dispute; and it is not even known for certain what disease it was - smallpox, exanthematous typhus, and bubonic plague have been suggested in modern times. Certainly the sources are unanimous n describing it as exceptionally destructive of human life. One of these source is the great doctor Galen, who was in Rome in 166, and left soon to return to his native Pergamum, to avoid the plague. But it has been modern scholars who have concluded that it was the most serious plague in the whole of antiquity, and a major factor in the decline of Rome. This view is probably exaggerated. But the effect of the plague was startling and severe, especially in the capital...and on the army, which, living in barracks, was particularly susceptible."
Anthony Birley, _Marcus Aurelius_ (2d ed., Yale University Press) 1987, pp.149-150.
The plague as a factor in the deline is open to debate, but in military/political terms the conquest of Seleucia and Ctesipon was the apex of the Roman Empire. Mesopotamia was abandoned in late 166.
john mage