Most empires have been destroyed by their trusted allies and vassals, and not by their enemies. The Persian Empire was destroyed 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. In 178 years, the Macedonians transitioned from vassals of the Persian Empire to its conquerers—Alexander the Great became the ruler of the Persian Empire.
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 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 Middle East and North Africa. 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 Arabs 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.
The Byzantine Empire was destroyed by its two allies—the Crusaders and the Ottoman Empire. The Crusaders had arrived in the Middle East to strengthen the Byzantine Empire but they sacked Constantinople. 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 were soon nibbling at Byzantine territory. In the fifteenth century, the Ottomans conquered the last outpost of the Byzantines: Constantinople.