Ardashir I in the context of Sassanid Persians

The Sasanian Empire (/səˈsɑːniən/), officially Eranshahr (Middle Persian: 𐭠𐭩𐭥𐭠𐭭𐭱𐭲𐭥𐭩 Ērānšahr, "Empire of the Iranians"), was an Iranian empire that was founded and ruled by the House of Sasan from 224 to 651 AD. Lasting for over four centuries, the length of the Sasanian dynasty's reign over ancient Iran was second only to that of the Arsacid dynasty of Parthia which immediately preceded it.

Founded by Ardashir I, whose rise coincided with the decline of Arsacid influence in the face of both internal and external strife, the House of Sasan was highly determined to restore the legacy of the Achaemenid Empire by expanding and consolidating the dominions of the Iranian nation. Most notably, after defeating Artabanus IV of Parthia at the Battle of Hormozdgan in 224, it began competing far more zealously with the neighbouring Roman Empire than the Arsacids had, thus sparking a new phase of the Roman–Iranian Wars. These efforts by Sasanian rulers ultimately led to the re-establishment of Iran as a major power of late antiquity.

The Sasanian dynasty (also known as the Sassanids or the House of Sasan), the last Persian lineage of rulers to achieve hegemony over much of Western Asia before Islam, ruled 224 CE–650 CE. the house of Sasan founded the Sasanian Empire of Iran, It began with Ardashir I, who named the dynasty in honour of his predecessor, Sasan.

The Shahanshah was the sole regent, head of state and head of government of the empire. At times, power shifted de facto to other officials, namely the spahbed. Upon the empire's conquest by the Islamic caliphate in 651, members of the imperial family fled in exile to China following the death of Yazdegerd III, where they would become accepted as members of the imperial court by Emperor Gaozong of Tang. Although there would be numerous attempts to invade Islamic Persia with Chinese support, this branch of Sasanids would remain in China indefinitely. Narsieh, grandson of Yazdegerd and last recorded Sasanid in China, would adopt the surname Li (李) in honor of the Chinese imperial family.

Artabanus IV, also known as Ardavan IV (Parthian: 𐭓𐭕𐭐𐭍), incorrectly known in older scholarship as Artabanus V, was the last ruler of the Parthian Empire from c. 213 to 224. He was the younger son of Vologases V, who died in 208.

Artabanus IV rebelled against his brother, Vologases VI, a few years after the latter succeeded their father as king. He was able to take control of most of the Parthian territories, although his brother continued to rule over a reduced principality. He fought off an invasion by the Roman emperor Caracalla in 217, preserving Parthian control over most of Mesopotamia. He then faced a rebellion in Pars by Ardashir, a local dynast. Artabanus's and Ardashir's armies met in April 224 at the Battle of Hormozdgan, where Artabanus was killed and his army was defeated. Ardashir then went on to conquer the rest of the Parthian Empire, inaugurating the rule of the Sasanian dynasty over Iran.

The Sasanian army was the primary military body of the Sasanian armed forces, serving alongside the Sasanian navy. The birth of the army dates back to the rise of Ardashir I (r. 224–241), the founder of the Sasanian Empire, to the throne. Ardashir aimed at the revival of the Persian Empire, and to further this aim, he reformed the military by forming a standing army which was under his personal command and whose officers were separate from satraps, local princes and nobility. He restored the Achaemenid military organizations, retained the Parthian cavalry model, and employed new types of armour and siege warfare techniques. This was the beginning for a military system which served him and his successors for over 400 years, during which the Sasanian Empire was, along with the Roman Empire and later the Eastern Roman Empire, one of the two superpowers of Late Antiquity in Western Eurasia. During the reign of Khosrow the Sasanian army underwent major reforms. The Sasanian army protected Eranshahr ("the realm of Iran") from the East against the incursions of central Asiatic nomads like the Hephthalites and Turks, while in the west it was engaged in a recurrent struggle against the Roman Empire.