Much has been said about the United States' "zero option" when it comes to its future presence in Afghanistan. But a rushed trip to Iran by the Afghan president to discuss a long-term partnership agreement shows that Kabul is considering its own options.President Hamid Karzai, who has resisted Washington's calls for him to sign a Bilateral Security Agreement (BSA) that would allow for a U.S. troop presence in Afghanistan after 2014, traveled to Tehran on December 8.Karzai made the trip with Washington showing no signs of bowing to his last-minute demands, and knowing that a security pact with the U.S. would only help secure Afghanistan's future in the short term.In the long term, Karzai must look to Afghanistan's neighbors, and that is where Iran has a role to play in terms of regional security, trade, and in determining the fate of the Afghan peace process.Graeme Smith, a senior analyst at the International Crisis Group in Kabul, says that in terms of security Tehran cannot match what Washington is offering as part of the BSA -- thousands of military trainers and billions in annual military and civilian aid through to 2024.But Smith says the Islamic republic's support could go a long way toward Afghanistan developing its aid-dependent economy and reaching a negotiated end with the Taliban."Iran will have a more long-term stake in the security and economy of Afghanistan than the United States ever will. So it makes a lot of sense for Karzai to build up a close relationship with his neighbor," Smith said.Afghanistan and Iran share deep historical, cultural, and linguistic links. And the sides moved to cement their burgeoning political and economic ties during Karzai's one-day trip to Tehran by agreeing to begin negotiations over a "Comprehensive Friendship And Cooperation Agreement."That deal would complement a separate border and security agreement signed between the two sides in August that commits both to fighting "terrorism, security related organized-crimes, as well as controlling the borders to fight human trafficking, (illegal) immigration, and drug smuggling."Even before the announced plans for a long-term pact, Tehran had shown it was a willing partner. Iran has contributed billions of dollars to the reconstruction effort in Afghanistan and has invested heavily in transport routes and Afghan businesses, especially in the west of the country.Tehran has plenty at stake. It is worried about the fate of the sizable Shi'ite Hazara minority that it has historically backed in Afghanistan. Tehran is also concerned about drug smuggling from Afghanistan. The influx of Afghan refugees into Iran after 2014 is another pressing concern. According to the United Nations, there are already at least 2 million undocumented Afghans living in Iran.While Kabul has welcomed the contribution Iran has made, it has also been wary. It is no secret that Tehran leaves its mark in Afghanistan in less obvious ways; for example, through its export of cultural and political views, strong media presence, and the funding of religious schools.Any underlying tensions, however, were shelved during Karzai's visit, during which Iranian President Hassan Rohani strongly backed Karzai's refusal to sign the BSA.Tehran is deeply suspicious of thousands of American troops remaining stationed to its east. Rohani requested that Kabul not sign the BSA, saying the pact is not in Afghanistan's long-term interests and would destabilize the region.Over the past decade, Karzai has been at pains to reassure Tehran over the American military presence. Even the draft text of the BSA that Karzai initially supported includes a clause stating that Washington under no circumstances can use Afghan soil to spy on, attack, or threaten its neighbors -- fears that Tehran has long held.Omar Samad, a senior Central Asia fellow at the New America Foundation and a former Afghan ambassador to France and Canada, says it is not surprising that Karzai has looked to bolster ties with regional actors amid heightening tensions with the United States.Apart from a trip to Iran, Karzai recently met with Pakistani Prime Minister Nawaz Sharif in Kabul and is expected to fly to India in the coming days.Samad says Karzai knows the importance of getting Afghanistan's historically intrusive neighbors on board."Afghanistan has been in a state of conflict for a very long time. This has created an opportunity for some of its neighbors to meddle in ways that have been destructive. So, Afghanistan has tried to come up with a new paradigm of good neighborliness," Samad said.With the tentative thaw in relations between Iran and the United States, there have been suggestions that Tehran could play a bigger role in Afghanistan.But Michael Kugelman, South Asia associate at the Woodrow Wilson International Center for Scholars in Washington, says the United States remains deeply suspicious of Tehran, and would be very uncomfortable with the idea of the Iranians taking on a larger role in Afghanistan.This particularly holds true, Kugelman says, in the event no U.S. troops remain in Afghanistan after 2014."Even though there is a thaw in bilateral relations between Iran and the United States now, it's still very early. It's a very volatile relationship and I don't think the U.S. is comfortable in the long-term looking to reconcile with Iran, even though things are looking good now," Kugelman said.