The Ancient Persian Empire, also known as the Achaemenid Empire, stretched from the Indus River Valley to Anatolia at its height. It was founded in the 6th century BCE by Cyrus the Great and expanded under subsequent rulers like Darius I. The empire dominated its region through military strength and established an extensive trade network along the Royal Road. Zoroastrianism became the dominant religion during this time. The empire began to decline in the 5th century BCE following its defeat by Alexander the Great and the Macedonians.