The McChrystal Report on Iran's hand in Afghanistan (emphasis added):
Iran plays an ambiguous role in Afghanistan, providing developmental assistance and political support to [Government of Afghanistan] while the Iranian Qods Force is reportedly training fighters for certain Taliban groups and providing other forms of military assistance to insurgents. Iran's current policies and actions do not pose a short-term threat to the mission, but Iran has the capability to threaten the mission in the future. Pakistan may see Iranian economic and political initiatives as threats to their strategic interests, and may continue to address these issues in ways that are counterproductive to the ISAF effort.
This is only the latest instance in which the U.S. military has publicly stated that Iran is arming and training Taliban fighters. It is worth remembering that Iran and the Taliban regime were on the verge of hostilities in 1998 after the Taliban slaughtered Shiites, including Iranian diplomats, in Mazar-e-Sharif. But that hasn't stopped the two from cooperating against their common enemy: America.
McChrystal says that Iran's role in Afghanistan is "ambiguous," by which he clearly means "capable of being understood in two or more possible senses or ways." Outwardly, the Iranians provide some nominal assistance with respect to building roads and other "developmental" projects. The Iranians are going to try to win influence however they can. But at the same time, they help the Taliban kill Americans because Americans inside Afghanistan are simply not acceptable to the mullahs.
As Steve Hayes noted, Obama's own Director of National Intelligence, Dennis Blair, made this clear in February of this year. Blair's declassified written responses to questions from the Senate Select Intelligence Committee included these comments (emphasis added):
"Iran has both long-term strategic and short-term tactical interests in Afghanistan and is not content with merely maintaining the status quo. In the short term, Iran is primarily concerned with preserving its national security and undermining Western influence in Afghanistan, which provides Iran's rationale for providing select Afghan insurgents with lethal aid. ...
Iran has not altered its activities in Afghanistan over the past year as various Iranian officials describe the Western presence as an occupation and Iran maintains a hostile relationship with the West. Iran's policy calculation in Afghanistan currently emphasizes lethal support to the Taliban, even though revelation of this activity could threaten its future relationship with the Afghan government and its historic allies within Afghanistan.
Iran is covertly supplying arms to Afghan insurgents while publicly posing as supportive of the Afghan government. Shipments typically include small arms, mines, rocket propelled grenades (RPGs), rockets, mortars, and plastic explosives. Taliban commanders have publicly credited Iranian support for their successful operations against Coalition forces."
The McChrystal Report says that Iran's training and arming of the Taliban does not currently jeopardize the mission. However, according to McChrystal, Iran does a pose a threat to the future of Afghanistan. "A number of risks outside of ISAF's control could undermine the mission, to include â€¦ actions of external actors such as Pakistan and Iran."
And, in the meantime, Iran's "lethal" support means more dead Americans and other coalition members.