The Achaemenid occupation of the Indus Valley decreased over successive rulers and formally ended around the time of the Alexander's conquest of Persia. This gave rise to independent kings such as Porus (ruler of region between Jhelum and Chenab rivers), Ambhi (ruler of region between Indus and Jhelum  rivers with capital at Taxila) as well as gaṇa sanghas or republics, which confronted Alexander during his Indian campaign circa 323 BCE. The Achaemenid Empire set a precedence of governance through the use of satrapies, which was further implemented by Alexander's Empire, the Indo-Scythians, and the Kushan Empire.