Tension between Tehran and the Taliban reached the boiling point last week when Iran deployed 70,000 troops on the border between the two countries. Underlying the conflict is the Sunni Muslim Taliban's fierce religious intolerance. "Taliban fighters are taught that Shiite Muslims are worse than infidels and should be killed," says McGirk. "If they repeat the massacres in Bamiyan that they're reported to have carried out in other areas they've captured, we could have a major war in the next couple of days."
If Iran is compelled to fight Afghanistan's Taliban militia, it may take a leaf out the U.S. playbook and wage war not directly, but by proxy. "Rather than risk the consequences of invading Afghanistan, Tehran will first try to reinvigorate the anti-Taliban opposition inside Afghanistan and among the 2 million Afghan exiles in Iran," says TIME New Delhi bureau chief Tim McGirk. That may prove difficult: It was reported on Monday that the Taliban had seized Bamiyan, the last opposition-held city inside Afghanistan. The fear in Tehran is that Taliban forces will massacre Shiite Muslim civilians in the occupied city -- a development that could spur Iran to immediate action.