What helped Darius the first to keep a huge country under control?

The very division of the Persian state into satrapy regions, with the satraps at the head of government, helped the Persian king effectively manage a huge territory. Satraps were endowed with extensive powers to collect taxes, maintain order, and improve the territories entrusted to them.

The Persians dealt with the uprisings quickly and brutally. The results of such reprisals were passed on by word of mouth. The purpose of such measures was to instill fear of the Persians in the subordinate peoples and to prevent repeated uprisings.