The Sassanid era began in earnest in 228, when the Shah Ardashir I destroyed the Parthian Empire which had held sway over the region for centuries. He and his successors created a vast empire which included those lands of the old Achaemenid Persian empire east of the Euphrates River. It was their long sought-after goal to reunify all of the old empire, and this brought them into frequent wars against the Roman Empire and Byzantine Empire.
Shah Khosrau II (Kasrā in Arabic) fleetingly achieved this goal in a series of wars against the Byzantine Empire between 602 and 616, conquering Egypt, Syria and Palestine. However, the Byzantine Emperor Heraclius turned the tide with a daring invasion of Persia itself. In 628, Khosrau was deposed with Heraclius' army at the gates of the capital of Ctesiphon. In the peace that followed, the Sassanids retreated to their traditional frontiers.
The long war exhausted both sides, and the Sassanids were soon destroyed by the rise of Islam.