Although all-out war appears unlikely, the tens of thousands of Iranian troops deployed in a threatening build-up along the Afghan border have drawn attention to the hostility between two of the world's most conservative Islamic governments.
To some non-Muslims in the West, the bearded Shiite Muslim clerics who govern Iran and the bearded Sunni Muslim Taliban militants who now control most of Afghanistan may appear indistinguishable.But they represent two wings of a great Islamic divide. And as the Taliban have consolidated their hold on power, imposing a strict vision of Islam across Afghanistan, Iranian leaders have responded with contempt and dismay.
They have accused the Taliban - whose restrictions, particularly on women, are far more rigid than those enforced in Iran - of giving Islam a bad name. They have provided support to the rival Afghan militia that is the Taliban's sole remaining foe.
And now there is the issue of the disappearance of scores of Iranians, including at least 11 diplomats, who were caught up in the fighting in Afghanistan last month. Iran has raised the prospect of taking military action against the Taliban unless reports that the Iranians were captured or killed can be quickly resolved.
Taliban leaders were quoted Sunday as saying that their followers were prepared to spill blood to fend off any incursion.
In interviews Sunday, Arab diplomats and other experts said they thought it unlikely that Iran really intended to mount a full-scale invasion.