Exercising his power of king through his generals.
Pony express
He built the Royal Roads
The governing system of the Persian Empire, resting on traditional local government, provincial government and central imperial government.
It was so extensive, of many peoples each with different customs.
The Royal Road or Achmaenid Road with its system of way stations and riders aided communication in Darius' empire. The road connected Susa with Persepolis and India.https://www.thoughtco.com/royal-road-of-the-achaemenids-172590
He consolidated local government, established Persian provincial governors in 20 provinces to oversee and protect them, and controlled them by himself and his council.
Self defence.
He divide the Persian Empire into provinces each headed by a governor called a "satrap." To encourage unity he hundreds of miles of roads built or repaired. It made it easier to communicate with different parts of the empire.
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Persian
It left the Persians with no legitimate king to unite opposition against his ongoing conquest of the Persian empire. He was then dealing with separate kingdoms and tribes.
He reorganized the government. He split the empire into two provinces called satrapies which were run by an official called a satrap.