The reasons for the military victories of the Persians?

The Persians in ancient times had a very strong and well-armed army. The dawn of the Persian state began in the middle of the 6th century BC. Then the Persians swiftly conquered one state after another. Then Persia was able to turn into a huge empire. In order to hold such vast territories, the Persian kings contained a large army of 400 to 800 thousand people. The army itself was divided into parts, which stood in separate satrapies.