Pakistan’s closure of one of the main NATO supply routes into Afghanistan and the string of attacks on NATO convoys transiting Pakistan over the last few days highlights the vulnerability of the entire coalition mission in Afghanistan to events inside Pakistan. Nearly eighty percent of supplies for the war effort in Afghanistan currently transit Pakistan. There have been several militant attacks on NATO trucks in the past but this is the first time Pakistan has formally closed down one of the border crossings.

The border closure demonstrates Islamabad’s furor over last week’s NATO strike that accidentally killed three Pakistani troops. NATO officials did not immediately issue an apology and instead claimed they had fired in self-defense. Islamabad is signaling the international community not to take for granted its sovereignty and cooperation with the Afghan war effort. NATO Secretary-General Anders Fogh Rasmussen’s official apology to the Pakistani Foreign Minister today should help soothe Pakistani anger over the incident.

The incident is similar to one that occurred in June 2008 in which a U.S. air strike on the border killed 11 Pakistani troops. Both of these unfortunate incidents point to the challenges of fighting an effective campaign against insurgents who cross freely back and forth along a porous Afghan-Pakistani border. While the 2008 incident raised tensions between the U.S. and Pakistan, it did not cause a major rupture in the relationship nor did it lead the Pakistanis to halt NATO convoys like they have this time.

The U.S. should focus on opening access routes outside of Pakistan to resupply its troops inside Afghanistan. Not only are the Pakistani supply routes increasingly under threat of militant attacks, the U.S. dependence on Pakistani supply routes provides Islamabad leverage to resist U.S. pressure to shut down Taliban sanctuaries and to crack down more forcefully on terrorist networks, like the Haqqani network, that attack coalition forces across the border and threaten the overall mission in Afghanistan.

Washington has begun to build up the so-called “Northern Distribution Network” through Russia, the Caucasus, and the Central Asian states and has already opened five supply routes from the countries north of Afghanistan, including Uzbekistan, Tajikistan and Turkmenistan. A more robust dialogue with Azerbaijan, Georgia, Kyrgyzstan, Russia, Tajikistan, Turkmenistan, and Uzbekistan regarding supply of Afghanistan and the future of Central Asian security could help the U.S. secure even more routes through the north, thus reducing its dependence on Pakistan.