Buddhism may have reached Balkh, now in Afghanistan, then under the Achaemenids, during or soon after the lifetime of the Buddha.
From the 2nd century Parthians such as An Shigao, were active in spreading Buddhism in China. Some of the earliest translators of Buddhist literature into Chinese were from Parthia.
The Sasanids persecuted the Buddhists when they came to power in AD 224 and promoted Zoroastrianism.
Surviving Buddhist sites were raided by the Ephthalites or White Huns, the nomadic confederation which at the height of its power (first half of 6th century) controlled territories in Transoxiana, Bactria, India, China.
Nevertheless, at the time of the Arab conquests, much of the eastern Iranian world was mainly Buddhist.
The Arab conquests brought the demise of Buddhism in eastern Persia and Afghanistan, but in some places, such as Bamiyan and Hadda (both Afghanistan), it survived until the 8th or 9th century.
In 1295 the Mongol ruler in Persia, Ghazan, was converted from Buddhism to Islam and made it the state religion of the Ilkhanate. He prohibited the practice of Buddhism, but allowed monks to go into exile in neighbouring Buddhist regions.