The success of the Persian Empire was a direct result of the Great Kings including Cyrus the Great, Cambyses II, and Darius. Together, they openly supported religious toleration and they formed an Immortal army.
The Persian Empire was tolerant, as well as efficient. They allowed the peoples to practice their own religions, customs, and methods of living life. There were many years of peace during the Persian Empire which facilitated trade and encouraged the well being of its people. Many eastern people owe gratitude for being subjects of the Great Kings of Persia, including Cyrus the Great, Cambyses, and Darius who governed and expanded the empire.
The formation of tribes in 735 BC allowed for a unified monarchy; however when Cyrus the Great unified the Persians in 559 BC, he went on the offensive against the Medes, making it the first Persian satrapy. He continued on to conquer the Greek city states, turned eastward, subduing the Iranian plateau, Sogdia, and western India. The east was secure, so in 539 BC he entered Mesopotamia and captured Babylon. He expressed extreme restraint, allowing governors to keep their present positions in office. He also issued an edict permitting Jews, who were brought to Babylon during the 6th century, to return to Jerusalem to rebuild the Temple. Cyrus the Great was viewed as "gentle" and expressed compassion for his people, even rebuilding temples and permitting religious toleration. He had created a "world empire" which allowed people to practice their own traditions, therefore upholding institutions.
After the death of Cyrus the Great in 530 BC, his son Cambyses II assumed power. Aided by the Phoenician fleet, he defeated the Egyptians and made Egypt into a province with Memphis as the capital. After his short reign, Darius emerged codifying Egyptian law and building a canal to link the Red Sea and the Mediterranean.
Under the reign of Darius, the Persians created the largest empire the world has ever seen including Egypt, Asia Minor, Mesopotamia, Thrace, and Babylon. In the Empire, it was divided into 20 satrapies for organization purposes. The satraps simply collected dues owed to the central government, raised military levies, and secured justice. They had little power overall. Communication was key, and it was said that there was a "Kings Eye", or a rather a person that kept a close eye on satrapies, as well as maintained roads that facilitated rapid transport of personnel. Like the Assyrians, the Persians established staying posts with fresh horses for the kings messengers.
By the time of Darius, the Persian monarchs had established a professional army, composed of contingents from various people making up the empire. At the core was 10,000 men that were infamously known as the Immortals because they never fell below that number. If one was killed, he was immediately replaced. Ultimately in battle, it was though they were invincible. The Persian Kings made excellent use of their cavalry, fighting behind enemy line. They also procured navy ships from subject states including the Phoenicians, Egypt, Ionian Greece, and the Anatolians.
At the same time, the spread of Zoroastrianism was due to its acceptance by Darius. Although Darius may have been a monotheist, drastic changes were still taking place as the Magi of Persia and the kings propagated Zoroaster’s teachings. Over time, however, Zoroastrianism lost its monotheistic emphasis, and the old nature of polytheism resurfaced. The Persian kings, being they were quite tolerant of other religions, encouraged gods and goddesses to take their rightful place in the Persian Pantheon.