How had Persia conquered so much territory?

Darius I ruled Persia from 521-486 BCE. He came to power after a possible palace coup though he was not the intended heir to the empire. He was the founder of the Achemenid Dynasty. His first few months in power were plagued with instability and unrest. Rebellions in Media, Elam, Babylon and Bactria threatened to break apart the growing empire. He had to fight in nineteen battles his first thirteen months in power to gain control and to ensure his hold on the leadership. Like his ancestors, he continued to expand the territory of Persian. In Egypt, he codified the laws, creating a sound leadership in the satrapy or province. He conquered the region of Thrace and strengthened his leadership. He encouraged large-scale building throughout the empire and advanced the boundaries of Persia to the Black Sea. He was also responsible for introducing coins as a means of exchange. He commissioned the minting of gold and silver coins creating a uniform monetary unit throughout the region. Your follow up question for the week: How had Persia conquered so much territory?