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The heartland of ancient Persia was in what is now southwestern Iran.
The Roman emperor Julian (also known as Julian the Apostate), who reigned from 361 to 363 AD launched a major invasion of the Sasanian Empire in 363 AD, aiming to decisively defeat the Persians and emulate Alexander the Great's conquests.
After initial successes, including reaching and winning a battle near the Persian capital Ctesiphon, his army faced supply issues and guerrilla harassment during the withdrawal. On June 26, 363, during a skirmish Julian was mortally wounded—likely by a spear or lance—while personally engaging in the fighting without full armor. He died shortly after from his injuries, at around age 31–32.
This makes him the clearest case of a Roman emperor dying in direct combat on Persian/Sasanian soil during an active campaign.
(Other Roman emperors met grim fates in Persian conflicts as well, but not quite the same way: e.g., Valerian was captured alive at the Battle of Edessa in 260 AD and died in captivity/humiliation in Persia, while Gordian III died on campaign against the Sasanians in 244 AD, but likely murdered by his own men rather than directly killed by the enemy in battle.)
Julian's death marked the end of his short reign and the last major Roman offensive deep into Persian territory for centuries, forcing his successor Jovian to accept unfavorable peace terms to extricate the battered army.