Afghanistan's neighbours have long felt threatened by events that they cannot control.
The longest borders are with Pakistan and Iran. Both nations have become the prime destinations for refugee populations during the two decades of conflict.
The United Nation's refugee agency says that there are 1.5m Afghans in camps in Iran - and 2m in Pakistan. Those numbers have been growing over the past week as tension has increased. Within Afghanistan there was an estimated 1m refugees on the move even before the current crisis.
Iran and Pakistan are now attempting to close the border amid fear of a regional meltdown.
Possible US Action: Any US military action requires the support of regional neighbours. Pakistan has come under intense pressure to allow the US to use, at the very least, its airspace. But Pakistan fears that it could face destabilisation from fundamentalists coming over the border from Afghanistan.
While there is no chance of any such arrangement with Iran, the US would be more than delighted with its neutrality. Routes into Afghanistan from the north are also problematic and would require the support of Russia which has forces stationed in Tajikistan.