Despite the fact that the land beyond the Euphrates had been swallowing up Roman armies at least since Crassus and 30,000 of his closest army buddies went walkabout and never came back, Roman emperors couldn't resist meddling in Persian politics.
On this day in 363AD, the Roman emperor Julian, his military reputation still surfing on the spankings he'd handed the Alamanni along the Rhine, marched his troops out of Antioch and into Sassanid Persia. It didn't really appear to be a "plan" kind of operation, as the army wandered around and dished out some random punitive damage to the Sassanid army and infrastructure before winding up camped outside of the capital at Ctesiphon.
Deciding against a lengthy siege, Julian decided to get all Xenophonic and march north, rather than back the way they'd come. He wound up taking a javelin through the liver during one of the innumerable skirmishes along the way, and the army was left in the hands of Jovian, a Serbian general who was no Alexander, if you know what I mean, and who some sources say was acclaimed emperor by accident. Shapur II soon brought the inept and harried Jovian to bay, and he signed away five provinces, three border forts, and a client kingdom to be named at a later date in order to be allowed to return to Roman territory and not take up a new career as Shapur's furniture.
The moral of the story? If you get a job working as a Roman emperor, stay on this side of the Euphrates.