This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Islam reaches Persia as Rashidun campaigns against the Sasanian Empire, beginning in 633, wear down and finally end more than four centuries of Sasanian rule. The campaigns close in 651 with the death of the last Sasanian king, Yazdegerd III, near Merv, the collapse of an imperial dynasty that had stood as one of the two great powers of the late antique Near East, opposite Byzantium itself.
This atlas records the honest shape of that eighteen year span deliberately: it is the collapse of an empire, fought and administered by Rashidun commanders across successive campaigns, that opened Persia to Islamic rule, not a simple or sudden story of conversion sweeping the region. Sasanian resistance persisted in pockets for years even as its central authority gave way, and Yazdegerd III's own death near Merv, far from the imperial heartland, marks the point that resistance finally had nowhere left to regroup.
What changed for Islam's spread is the fall of a rival empire rather than another province of a single Byzantine adversary. With Sasanian Persia gone, the caliphate that had already absorbed Syria and Egypt now held the other great power of the pre Islamic Near East as well, a reach few observers in Muhammad's own lifetime could have anticipated.