A U.S. drawdown would almost certainly reorient Iran’s approach to its neighbor to the east. Many Americans don’t know that the fundamentalist Sunni-Taliban government of Afghanistan and the orthodox Shia government of Iran came to the brink of war in 1998. The Taliban repressed Afghan Shiites, many of whom live in the western part of the country, near the Iranian border. At the same time, Iran provided arms and financial assistance to the “northern alliance” of Afghan Tajiks and Uzbeks, who never surrendered to the Taliban government. The Taliban, in turn, received strong backing from Pakistan. Within Pakistan, sectarian attacks on Shia were and remain quite common.

All of these factional tensions persist to this day. As many have observed, a Sunni-Shia civil war remains interwoven with conflicts across the greater Middle East. Were the United States to significantly reduce its support to the current government of Afghanistan, Iran would likely find it in its own interest to cease its reported flirtations with the Taliban and lend support to the Afghan government, or to broker a settlement between the two. Iran is interested in building a new “silk road” trading route that runs from Central Asia to the Persian Gulf, which could best flourish in a peaceful Afghanistan. Moreover, Iran would probably find it reasonable to station more military forces on its eastern border to deter Taliban misbehavior. Overall, an increase in Taliban influence is ultimately a threat to Iran’s security, and would place a new constraint on Iran’s adventurism elsewhere in the region, where it typically seeks gains at the expense of U.S. allies such as Saudi Arabia and Israel, who inevitably come to Washington demanding assistance to shore up their positions.