Hephthalite Empire
In 572, Justin II, the successor of Justinian, broke the peace treaty and sent a Byzantine force into the Sasanian region of Arzanene. The following year, Khosrow besieged and captured the important Byzantine fortress-city of Dara, which drove Justin II insane. The war would last till 591, outliving Khosrow. Khosrow's wars were not only based in the west. To the east, Khosrow, in an alliance with the Göktürks finally put an end to the Hephthalite Empire, which had inflicted a handful of defeats on the Sasanians in the 5th-century, killing Khosrow's grandfather Peroz I. To the south, Iranian forces led by Wahrez defeated the Aksumites and conquered Yemen.