Orontid and Artaxiad
After the fall of Urartu around 585 BC, the Satrapy of Armenia was ruled by the Armenian Orontid Dynasty, which governed the state in 585–190 BC. Under the Orontids, Armenia during this era was a satrapy of the Persian Empire, and after its disintegration, it became an independent kingdom. During the rule of the Orontid dynasty, most Armenians adopted the Zoroastrian religion.
After the destruction of the Seleucid Empire, a Hellenistic Armenian state was founded in 190 BC. It was a Hellenistic successor state of Alexander the Great's short-lived empire, with Artaxias becoming its first king and the founder of the Artaxiad dynasty (190 BC–AD 1). At the same time, a western portion of the kingdom split as a separate state under Zariadris, which became known as Lesser Armenia while the main kingdom acquired the name of Greater Armenia.
During the reign of Tigranes the Great (95–55 BC), the kingdom of Armenia was at the zenith of its power and briefly became the most powerful state to the Roman east. Artaxias and his followers had already constructed the base upon which Tigranes built his empire. Despite this fact, the territory of Armenia, being a mountainous one, was governed by nakharars who were largely autonomous from the central authority. Tigranes unified them in order to create internal security in the kingdom. The borders of Armenia stretched from the Caspian Sea to the Mediterranean Sea. At that time, the Armenians had become so expansive, that the Romans and Parthians had to join forces in order to beat them. Tigranes found a more central capital within his domain and named it Tigranocerta.
Large territories were taken from Parthians, who were forced to sign a treaty of friendship with Tigranes. Iberia, Albania, and Atropatene also lost territories and the remainder of their Kingdoms became vassal states. The Greeks within the Seleucid Empire offered Tigranes the Seleucid crown in 83, after which the Armenian empire reached as far south as modern Acre, Israel resulting in a conflict with Hasmoneans.
The Third Mithridatic War and defeat of the King of Pontus by Roman Pompeius resulted in the Kingdom of Armenia becoming an allied client state of Rome. Later on, in 1 AD, Armenia came under full Roman control until the establishment of the Armenian Arsacid dynasty. The Armenian people then adopted a Western political, philosophical, and religious orientation. According to Strabo, around this time everyone in Armenia spoke "the same language."
From Pompeius' campaign Armenia was, for the next few centuries contested between Rome and Parthia/Sassanid Persia on the other hand. Roman emperor Trajan created even a short-lived Province of Armenia between 114-118 AD.
Indeed, Roman supremacy was fully established by the campaigns of Gnaeus Domitius Corbulo, that ended with a formal compromise: a Parthian prince of the Arsacid line would henceforth sit on the Armenian throne, but his nomination had to be approved by the Roman emperor.
Because this agreement was not respected by the Parthian Empire, in 114 Trajan from Antiochia in Syria marched on Armenia and conquered the capital Artaxata. Trajan then deposed the Armenian king Parthamasiris (imposed by the Parthians) and ordered the annexation of Armenia to the Roman Empire as a new province.
The new province reached the shores of the Caspian sea and bordered to the north with Caucasian Iberia and Caucasian Albania, two vassal states of Rome. As a Roman province Armenia was administered by Catilius Severus of the Gens Claudia. After Trajan's death, however, his successor Hadrian decided not to maintain the province of Armenia. In 118 AD, Hadrian gave Armenia up, and installed Parthamaspates as its "vassal" king.
Armenia, under its Arshakuni dynasty, which was a branch of the eponymous Arsacid dynasty of Parthia, was often a focus of contention between Rome and Parthia. The Parthians forced Armenia into submission from 37 to 47, when the Romans retook control of the kingdom.
Under Nero, the Romans fought a campaign (55–63) against the Parthian Empire, which had invaded the kingdom of Armenia, allied to the Romans. After gaining (60) and losing (62) Armenia, the Romans under Gnaeus Domitius Corbulo,legate of Syria entered (63) into an agreement of Vologases I of Parthia, which confirmed Tiridates I as king of Armenia, thus founding the Arshakuni Dynasty.
The Arsacid dynasty lost control of Armenia for a few years when emperor Trajan created the "Roman Province of Armenia", fully included into the Roman Empire from 114 to 117 AD. His successor Hadrian reinstalled the Arsacid Dynasty when nominated Parthamaspates as "vassal" king of Armenia in 118 AD.
Another campaign was led by Emperor Lucius Verus in 162–165, after Vologases IV of Parthia had invaded Armenia and installed his chief general on its throne. To counter the Parthian threat, Verus set out for the east. His army won significant victories and retook the capital. Sohaemus, a Roman citizen of Armenian heritage, was installed as the new client king.
The Sassanid Persians occupied Armenia in 252 and held it until the Romans returned in 287. In 384 the kingdom was split between the Byzantine or East Roman Empire and the Persians. Western Armenia quickly became a province of the Roman Empire under the name of Armenia Minor; Eastern Armenia remained a kingdom within Persia until 428, when the local nobility overthrew the king, and the Sassanids installed a governor in his place.
According to tradition, the Armenian Apostolic Church was established by two of Jesus' twelve apostles—Thaddaeus and Bartholomew—who preached Christianity in Armenia in the 40s—60s AD. Between 1st and 4th centuries AD, the Armenian Church was headed by patriarchs.
In 301, Armenia became the first nation to adopt Christianity as a state religion, amidst the long-lasting geo-political rivalry over the region. It established a church that today exists independently of both the Catholic and the Eastern Orthodox churches, having become so in 451 after having rejected the Council of Chalcedon. The Armenian Apostolic Church is a part of the Oriental Orthodox communion, not to be confused with the Eastern Orthodox communion. The first Catholicos of the Armenian church was Saint Gregory the Illuminator. Because of his beliefs, he was persecuted by the pagan king of Armenia, and was "punished" by being thrown in Khor Virap, in modern-day Armenia.
He acquired the title of Illuminator, because he illuminated the spirits of Armenians by introducing Christianity to them. Before this, the dominant religion amongst the Armenians was Zoroastrianism. It seems that the Christianisation of Armenia by the Arsacids of Armenia was partly in defiance of the Sassanids.
In 405-06, Armenia's political future seemed uncertain. With the help of the King of Armenia, Mesrop Mashtots, a unique alphabet was created to suit the people's needs. By doing so, he ushered in a new Golden Age and strengthened Armenian national identity.
After years of rule, the Arsacid dynasty fell in 428, with Eastern Armenia being subjugated to Persia and Western Armenia, to Rome. In the 5th century, the Sassanid Shah Yazdegerd II tried to tie his Christian Armenian subjects more closely to the Sassanid Empire by reimposing the Zoroastrian religion. The Armenians greatly resented this, and as a result, a rebellion broke out with Vartan Mamikonian as the leader of the rebels. Yazdegerd thus massed his army and sent it to Armenia, where the Battle of Avarayr took place in 451. The 66,000 Armenian rebels, mostly peasants, lost their morale when Mamikonian himself died in the battlefield. They were substantially outnumbered by the 180,000–220,000-strong Persian army of Immortals and war elephants. Despite being a military defeat, the Battle of Avarayr and the subsequent guerilla war in Armenia eventually resulted in the Treaty of Nvarsak (484), which guaranteed religious freedom to the Armenians.
With the partition of Armenia in 387 by the Byzantines and Sassanids, the western half became part of the Byzantines known as Byzantine Armenia, while the eastern (and much larger half) became a vassal state within the Sassanid realm.
In 428, the Arsacid dynasty of Armenia was completely abolished by the Sassanid Persians, and the territory was made a full province within Persia, known as Persian Armenia. Persian Armenia remained in Sassanid hands up to the Muslim conquest of Persia, when the invading Muslim forces annexed the Sassanid realm.