When the Taliban rose to power in the 1990s, it was greeted with hostility by the neighboring Islamic Republic. The Shiite mullahs who ruled Iran were particularly enraged by the Sunni mujahideen’s vicious persecution of Afghan Shiites. But more recently, explains Dore Gold, there have been signs of a somewhat more cooperative relationship:
The Iranians [have] pursued a strategy of supplying Taliban units with arms and cash as well as training Taliban fighters. . . . Iran was employing Shiite Afghans in Syria as well in order to advance Iranian interests in the Levant. They were used to promote Iran’s war against [rebel groups] on Syrian territory. But they also could provide an important force multiplier in Syria for Iran in a future war against Israel.
Both states were committed to seeing U.S. power in Afghanistan weakened. That joint interest should have pulled both countries into greater cooperation. But what will happen after the U.S. has withdrawn from Afghanistan? Does there remain any basis for Iranian-Afghan cooperation against American power once it is gone? What is more likely is that Iran will resume its policy of expansionism towards Afghanistan that it has demonstrated towards the Middle East as a whole in recent years.
There were also historical factors. When Persia—then known as the Safavid empire—officially made Shiism its state religion in the 16th century, its borders extended well beyond Iran’s present-day frontiers. In the east, the Safavid empire stretched to what is today the Afghan city of Herat. It should come as no surprise that one of the main languages of Afghanistan, Dari, is a dialect of Farsi, the Persian language. . . . Recovering lost Persian territories has been a theme of Iranian policy towards the Arab world and could well serve as a motive for the Iranians in their relations with their eastern neighbors, as well, especially Afghanistan.
Read more on Jerusalem Center for Public Affairs: https://jcpa.org/article/iran-and-the-taliban-bitter-enemies-or-potential-partners/