Final answer:
The Persian Empire united a vast territory under a single government by dividing it into twenty satrapies, establishing a network of roads for efficient communication, standardizing laws and coinage, and using monumental architecture and art to project power.
Step-by-step explanation:
The Persian Empire managed to unite a vast and diverse terrain under a single centralized government through various administrative and infrastructural reforms. First, under the reign of Darius I, the empire was divided into twenty satrapies or provinces, each governed by a satrap who held both civil and military authority, albeit under the watchful eyes of another military general and itinerant royal inspectors known as the 'eyes and ears of the king.' This structure created a balance of power and a way for the central government to maintain control over the expansive territories.
To ensure efficient communication across the large empire, which stretched from the Indus River valley to Libya, the Persians continued the Assyrian practice of building highways for their messengers, most notably the Royal Road, facilitating the rapid delivery of messages and orders. Laws were standardized, and coinage was issued to implement a robust economic system.
Additionally, monumental architecture and art were employed by Persian kings to project power and legitimize their rule. The Achaemenid Empire, as it is also known, thrived by integrating local customs into the imperial culture while retaining a strong centralized bureaucracy. The modest taxation system and the strategic method of mobilizing armies based on regional strengths further sustained the unity and strength of the Persian Empire until its eventual conquest by Alexander the Great.