Most superpowers were annihilated by their trusted allies and vassals, and not by their enemies. The Persian Empire was annihilated by its former vassal, the Kingdom of Macedonia. In the sixth century, the Macedonians were a minor tribe in Europe. The Persian Emperor Darius I made them the legitimate satrap of Macedonia. The Persians gave military training to the Macedonians and armed them, so that they could serve in the Persian army as mercenaries.
The Western Roman Empire was conquered by its former vassals, the Visigoths (who emerged from the Gothic groups in the fourth century AD). The policy of using barbarians in the Roman army was introduced by Augustus. By the third century AD, the Roman army was very diverse—it had a large number of barbarian soldiers. In the fourth and fifth centuries AD, Goths played an important role in defending Rome against the Hun army. But in 410 AD, the Goths sacked Rome and conducted a great massacre. In 476 AD, the Goth King Odoacer captured the Roman imperial throne, bringing the Western Roman Empire to an end.
The Byzantine Empire played an important role in the rise of Arab Islamic power in the Levant. They funded, armed, and trained the early Arab warriors because they wanted to use the Arabs against their enemy, the Second Persian Empire (the Sassanid Empire). But after conquering the Second Persian Empire, the Arab movements turned their eyes towards the territories of the Byzantines. The Arab army first besieged Constantinople in 674–678 and then in 717–718. Both sieges failed but the Byzantine Empire’s reputation was shattered in the Levant.
The Byzantine Empire was destroyed by two of its allies—the Western Crusaders and the Ottoman Empire. The Western Crusaders had arrived in the Levant to strengthen the Byzantine Empire but they did more to destroy it than any Islamic Kingdom in the Levant. The Ottoman Sultans began as allies of the Byzantines in the thirteenth century. The Byzantines helped the Ottomans in expanding their power in Anatolia in exchange for using Ottoman mercenaries in their army. But the Ottomans started nibbling at Byzantine territory. In the fifteenth century, the Ottomans conquered the last outpost of the Byzantines: Constantinople.
Here’s a brief account of the evolution of the Macedonian vassalage to the Persians:
In 512 AD, Darius I, King of Persia, began his campaign to suppress the Scythian nomadic tribes who were destabilizing the outlying areas of his empire. At that time, Amyntas I of Macedon, ancestor of the future Alexander the Great, became a vassal of the Persian Emperor, and Macedon became a part of the Persian Empire (the Achaemenid Empire). Amyntas offered the homage of earth and water to Megabazus (the general of Darius I) in a gesture of submission to Persia. A large number of Macedonians joined Darius’s army which chased the Scythians into Ukraine, the tribe’s original homeland, and razed their farmlands and habitations.
Amyntas’s successor, Alexander I, continued to be a vassal of the Persian Emperor. He gave his sister in marriage to Persian general Bubares to cement his bond with the Persian Empire. He was the ruler of Macedon when Xerxes became the King of Persia in 486 BC. The Macedonians were a part of the Persian Army which invaded Greece in 480 BC. When the Persian navy was defeated in the Battle of Salamis, Alexander I played an important role in the peace negotiations. Herodotus has mentioned Alexander I several times. He says that Alexander I had the title of hyparchos (viceroy) and that he was constantly by the side of Xerxes to take his commands.
After the end of the Persian war in Greece, Xerxes granted a significant amount of autonomy to Alexander I in Macedonia. Several generations later Philip II acquired the throne of Macedonia. He was a military genius. He gained full independence from the Persian Empire and created an army of 50,000 soldiers. His son Alexander the Great used this Macedonian army in 334 BC to invade and conquer the Persian Empire. It had taken the Macedonians 178 years to transition from vassals of the Persian Empire to its conquerors.
When a superpower falls, it is not shown any mercy by its civilizational rivals. To establish itself as a superpower, a civilization has to commit a lot of virtuous acts, courageous acts, and brutal acts. When it is at the peak of its power, people of other cultures recognize the superpower’s acts of virtue and courage, and they admire it, but when it falls, they remember only its acts of brutality, and they despise it. They want to punish it for its past brutality—this leads to several excesses. When a superpower falls, it is completely crushed, and it never rises again.