The Biggest Empire, which the ancient world has ever seen, would be the Achaemenid Persian Empire. This empire protracted from Anatolia, Egypt, Asia, northern India and Central Asia. Making of this empire began in 550 B.C. The Achaemenid is known as the first Persian Empire and the largest. Ancient history basically tells us Zoroastrianism developed somewhere in Iran about 1000 years after Zoroaster. Scholars have said the Achaemenids era had finally began when Cyrus the great successfully went against his father in law who was ruler of the Medians. That is when Zoroastrianism started to influence the Median tribes. The Median then began to move eastwards past the Zagros mountains. They moved to Kermanshah and began to connect with Zoroastrians. Within the same time period in the southern Iran Persians moved towards the East. The Persians then started to overpower a tribe called the Elamites. While this was taking place Zoroastrianism moved forward among all Medians and Persians which were moving towards the East. Little about Zoroastrianism was known until around 549-550 BCE. When Cyrus the great who lead the Persians founded Western Iran and created the first biggest Persian Empire. The Achaemenid period developed around 558 B.C by Cyrus the great. The Zoroastrian religion then united forces with the Persian Empire. No one actually knows what happen to the Zoroastrian community after Zoroaster passed away. As for his message it was said that it had eventually gone all across the Iranian world. It is not certain how long it had taken for Zoroastrianism to become the Iranians main religion. Zoroastrianism had gone through the Iranian worlds Western part by the 6th century BCE. The Iranian people practiced their traditional Iranian religion up until the beginning of the Achaemenid Empire, on which they had converted to Zoroastrianism. Cyrus king of the Acheamnids Empire had replaced the Median Empire with the Achaemenid Empire.
Bibliography: M. Boyce, ‘’ Zoroaster and His Teachings,’’ in Zoroastrians: Their Religious Beliefs and practices. London, 2001 : 48-77 http://www.iranchamber.com/history/articles/achaemenid_society_culture.php http://www.cais-soas.com/CAIS/Religions/iranian/Zarathushtrian/zoroastrianism_under_achaemenids.htm http://www.iranicaonline.org/articles/achaemenid-religion http://www.iranchamber.com/religions/articles/zoroastrianism_under_achaemenids.php