Due to the combination of various factors, the Roman state was overwhelmed and it failed.

The uncertain foreign war against the powerful countries in the East and the Middle East destroyed Rome's economy. At the same time, the rising military forces in the north and west were firstly various Germanic tribes and nomadic Huns, and then newly baptized Slavic countries and Norman adventurers opened up space for the western territory of the empire in the turmoil.

Roman politics from the former Trinity to the last Korean dynasty was often plagued by internal conflicts and civil wars, which greatly weakened the whole country.

These conflicts, in turn, led to urbanization, trade collapse, autonomy of provinces and officials, and a sharp drop in tax revenue, further weakening the country in a vicious circle.

Because this will be a long answer, we can deeply study the history of the Roman Empire (and later the Eastern Roman Empire). Let's sum up the following points that led to the decline of the two empires: the powerful neighbors in the east were always hostile to the Romans, the nomadic and barbaric tribes in the north invaded, frequent civil wars and economic depression.

It is difficult to maintain the stability of the empire when one is constantly at war with another superpower in the world.

Three superpowers in the late ancient times: Rome under Jorio Claudia (left), Persia under Parthia (middle) and China under Han (right).

Although it is generally believed that the Roman Empire officially collapsed when Odoacer plundered Rome in the 5th century, it survived in a reduced form in the East because the Eastern Roman Empire centered on Constantinople.

Even after the decline of the Western Roman Empire, people and countries in the East continued to call the eastern half the Roman Empire and the Romans (in non-Byzantine terms).

Therefore, before continuing, it is important to clarify that the Roman Empire did not end in 476, but when 1453 Constantinople fell into the hands of Ottoman Turks. It lasted for more than 1000 years, which greatly changed how we should look at this situation.

Artist's Re-creation of the Fall of Constantinople (1453) Why couldn't the Middle East Empire be defeated during Roman rule? In the early days, Rome's tactics and strategies enabled it to defeat Diadochi, Alexander's successor, during its expansion.

Some of these countries undoubtedly adopted Middle East strategies and tactics in the war, not to mention inheriting Alexander's marriage legacy.

So, why was it so difficult for the Romans to attack the later Eastern Empire, such as Parthians, Sassanians, and later Arabs and Turks? At that time, Alexander was able to conquer the entire Persian empire, which was bigger and more powerful than its successor countries.

Diadochi originated from logistics and communication some time before Rome's eastward expansion.

Even in the heyday of Trajan, the Romans lacked the ability to project power far from the border. This is not a problem of opposing separatist Germanic and Iberian tribes, but a problem of an organized developed country, which has a huge heartland to rely on.

In fact, Persians often return to the Iranian plateau, where they reorganize new troops and fight back. Darius Iii tried to do the same thing after he lost to Alexander in the Battle of Guaga and Milla, but he was murdered. In treacherous hands, the empire fell into chaos and Alexander occupied the whole country.

The only way for Rome to achieve meaningful conquest is to reproduce a similar situation: crushing Persians with overwhelming victory and occupying Mesopotamia, weakening the Persian government in this way and forcing a favorable peace treaty to be signed.

But history shows that the Parthians and Sassanian rulers at that time were unwilling to give up or even accept peace, while Mesopotamia was in the hands of the Romans, so the Romans often overextended themselves and lost peace.

During the Roman-Persian War, the two sides could not carry out meaningful conquest, which made the conflict last for six centuries (53 BC to 628 AD).

The Roman army may have gone deep into Mesopotamia and conquered it (they fired at the Persian capital many times), but the hostile population and encirclement of the Persian army and its allies often did not trap the Romans, but were defeated by hunger.

The same was true of the Persian invasion of Roman territory, and the Romans often checked their progress in the depths of Syria. Therefore, the whole eastern border has always been a high priority of the Roman emperor, and until the Eastern Roman Empire remained the most populous and powerful place on its border.

The continuous war between the Romans and Persians drained the treasury of the empire. Due to the constant threat of invasion, Rome had to maintain a large number of standing troops and fortifications on the border, which dealt a heavy blow to the economy.

The Persian border often caused trouble to the Roman emperor, but it was not worth the loss.

Moreover, this does not mention the fact that the two superpowers are bound to conflict: Persians continue to claim that the eastern part of Rome belongs to the old Persian Empire, so this is their natural route, while ambitious or greedy Roman emperors often cannot resist the temptation to plunder the rich in Mesopotamia.

When your enemies think that half of your land belongs to them, it is difficult to achieve peace (map of Achaemenid Empire, the first Persian Empire)

Therefore, the serious national treasury and Persian hostility meant that the Romans had to disperse their remaining forces to other borders. In peacetime, this is not a problem. By concentrating their troops on specific fronts, it is easy to issue local threats. It became a problem only in the later period of imperial history, that is, in the 50 years from 235 to 284 AD, which was called "the first crisis of the third century".

This crisis is essentially the climax of all the events listed in the introduction: war against Sassanian Persia, civil war, barbaric invasion, economic depression and incompetence of the central government.

It began during the reign of Alexander Severus (AD 222-235), when new hostilities broke out between Rome and Persia, just as Germanic tribes attacked Roman territory along the Rhine and Danube borders.

In order to end the Germanic invasion as soon as possible in response to the Persian invasion, Severus personally made a peace treaty and bribed the chiefs to stop attacking temporarily. However, this led to the emperor's great prestige, especially in the army. He believed that the attack on Roman territory must face severe retaliatory measures.

Generals and troops also believe that the Roman emperor, in terms of his rank, needed to solve problems through force rather than diplomacy in order to maintain the emergence of power. Severus's repeated failures to Persians and barbarians and the necessity of using diplomatic means further worsened his image.

Dissatisfaction quickly spread to what they thought was a weak position in the army.

This disobedient army later killed Alexander Severus, who was in Germany at that time? Run for his puppet emperor Maximinu and crown him.

This set a dangerous precedent in the Roman legion: arrogance caused by their right to choose an emperor at will. This danger continues to hang over the future Roman emperor, just like the shroud, indicating that the war with Persia is still going on.