Over the summer of 1998, the Northern Alliance lost Mazar-i-Sharif to the Taliban. During the taking of the city, the Iranian Consulate was attacked. Eight staff members were killed. Additionally, a journalist also lost his life. Iran claimed that the Taliban had taken another 70 Iranians, mostly truck drivers, prisoner as well. At the time, the Iranian leadership grew even more angry over the whole situation when they came forward with what was claimed to be irrefutable proof that the attack on the consulate had not only been deliberate, but had been ordered by the Taliban's leader, Mullah Omar.
OTL, the Iranians whipped up public support and then moved 70,000 troops to the border. There they exercised for a planned invasion. Between September and November, the captives were released and the situation at least cooled down. So let's run the WI: the Taliban were extremely obstinate with the US, let's just give them the same Brain Eater attack with the Iranians. We've beaten the Persia horde time and again (true or not): let them come!
September 1998, 70k Iranian troops swarm across the Herat (IIRC). If they are half way competant, I suspect that they'd roll up the Taliban without a sweat. They'd prolly not endear themselves to the locals, esp. the Pashtuns/Pathans, because the differences of Sunni vs. Shia. Also, when the guerilla attacks take place, the Iranians are going to pull no punches. The interesting question is "How will Pakistan respond?" The Taliban of OTL were at least in part supported by members of the Pakistani intelligence agencies. A Shia run Afghanistan is probably not something that the Pakistanis want to see. At all.
This also throws interesting butterflies into everybody's favourite terrorist's plan: does it get disrupted? Does this even effect it? After all he might just relocate to Sudan or Somalia or elsewhere. So WI?