 Complete the following phrase:  The Achaemenid Empire, established in 550 BC after the fall of the Median Empire, rapidly expanded over the course of the sixth century BC. Asia Minor and the Lydian kingdom was conquered in 546 BC, the Neo-Babylonian Empire in 539 BC, Egypt in 525 BC and the Indus region in 513 BC. The Achaemenids employed satrapal administration, which became a guarantee of success due to its flexibility and the tolerance of the Achaemenid kings for the more-or-less autonomous vassals. The system also had its problems; though some regions became nearly completely autonomous without any fighting such as Lycia and Cilicia, other regions saw repeated attempts at rebellion and secession. Egypt was a particularly prominent example, frequently rebelling against Achaemenid authority and attempting to crown their own Pharaohs. Though it was eventually defeated, the 