On March 9, 590, Kuslu II became the Sassanid monarch of Persia.

The Sassanid family rose in Fars ("Persia" was originally the ancient name here) in southwestern Iran. The first ancestor of the dynasty, Sassan, was a Zoroastrian priest. At the beginning of the 3rd century, his son Papek established himself as king according to his base of Staher (now Istakhr). In 224, Papek's son Ardashir I (reigned 224-241) overthrew the Parthian dynasty (Arrest Empire), and in 226 he was crowned at Taisi, claiming to be the king of kings. He went on to occupy a large area of the original Parthian Empire and established a centralized Sassanid Persian feudal empire. He also invaded Armenia north, breaking the alliance between the Parthian family and the Dazuki, and consolidating the borders of the empire. He made Zoroastrianism the state religion.

Shapur I (reigned 241-272) dealt three severe blows to the Roman Empire. The third great defeat of the Roman army near Edessa and the capture of the Roman emperor P.L. Valerian (260). He ruled a territory of 27 provinces, as far east as the present-day Syr Darya and the upper reaches of the Indus River. Shapur I called himself the king of kings in Iran and non-Iran. In order to rule a multi-ethnic empire, he established the world religion founded by Manichaeism. After his death, Manichaeism was persecuted.

In the 4th and 5th centuries, the Sassanid dynasty and its western neighbor, the Byzantine Empire, continued to fight wars, winning and losing each other; in the east, they were threatened by nomadic people - a group of Hunnites and a group of Hun people. King Firuz of the Sassanid (also known as Belus, reigned 457-484) resisted the Quda and died in battle. At that time, the royal family was weakened and the nobles were tyrannical. Cavard I, son of Firuz (488-496, 499-531), attempted to use the Mazik movement to suppress the tyranny of the nobles, and was overthrown by the nobles in 498. In 501 he was restored to the throne with the support of the Quda.

During the rule of Kusruu I (531 - 579), son of Kawad I, the empire reached its peak. Three years before taking the throne, he colluded with Zoroastrian monks to suppress the Mazdak sect; after taking the throne, he implemented a series of reforms. Increase national treasury revenue, ease domestic conflicts, and rely on small aristocrats to hold local administrative positions, thereby reducing the political role of a few aristocratic families. In about 558, he allied with the Western Turks and defeated Koda. In 572, in order to cut off Byzantium's access to China silk from the sea, Yemen was occupied. Later, Byzantium joined forces with the Western Turks to launch a long-term war against Iran.

Kuslu I's son, Holmezd IV (reigned 578-590), continued the war against the Byzantines. At home, he tried to strengthen his position by relying on Christians and continued to implement a policy of suppressing the great aristocracy, thus angering the great aristocracy and the Zoroastrian monks, and was overthrown. His son Kuslu II (reigned 590, 591-628) soon succeeded the throne. In 602, he waged war against Byzantium, conquered all of Asia Minor, sacked Syria, occupied Antioch, Damascus, Jerusalem, and finally Egypt in 619, from which he received the title Parvïz (meaning victor).

In 622, Byzantine Emperor Heraclius recovered Asia Minor and entered the middle reaches of the Tigris River in 627.

Kuslu's invasion of Yemen upset the balance of the political and social structure of the peninsula, which played a role in promoting the reshuffling of Arab society. More dramatically, the burst of energy from this change eventually led to the overthrow of the Sassanid dynasty. Muhammad founded Islam and fought in the south and north. In just over 20 years, the scattered Arabian Peninsula has been unified under the banner of Allah. Then the Muslims conquered everywhere, and the Persian Empire was also included in the list of conquests by them. In 637, the Arab army, led by Saird bin Abi Wagas, defeated the Sassanid army in Qadisiya and captured Tessiphon. At this time, only 62 years after Kuslu conquered Yemen. The battle of Nehavander in 642 decided the fall of the Sassanid Empire. The last king, Yizi Zuo III, was killed by a miller at Mulu in 651. His son Pyrus fled to the Eastern Tang Dynasty, where he was a general of Youwu Wei, when the Tang Dynasty was ruled by Emperor Gaozong of Tang.

Review: Monarchs during the period of imperial expansion