Along the 100 miles of desert that separate this province from Pakistan, there is only one, lonely outpost of police who guard the border.
There are only a few dozen officers responsible for making sure weapons and supplies for the Taliban do not enter Afghanistan from Pakistan. There are no U.S. soldiers within 30 miles.
After 7½ years of war, the border of Zabul on Afghanistan's side and Baluchistan on Pakistan's side is so porous and so out of reach for the few U.S. troops in this province that American officials don't even know what occurs there.
For a map of the Afghan border with Pakistan's Baluchistan province, click here.
The problem is not limited to Zabul. From the point where Pakistan, Iran and Afghanistan meet in the west, through the vast desert that spreads across southeast Afghanistan and southwest Pakistan, U.S., Pakistani and Afghan officials admit there are not enough troops or police to properly secure a border that militants cross easily.
Despite the lack of attention, the border's security is essential for the future of the war. If it is not secured, U.S. officials say, there is no way to cut off the supply lines for the Taliban who have made southern Afghanistan the most dangerous area in the world for U.S. troops.
This story is part of an ABCNews.com series "The Fight for Afghanistan, Where We Stand." The complete series can be found on this site's News page.
"The Zabul border supply line/rat line was one of the first or one of the most significant … supply lines from Pakistan," says Maj. Greg Cannata, the lead military officer in Zabul. "And to not counter that and to not be able to read it and not be able to get ahead of it, that hurts. That hurts what we're trying to do in the key population areas along [the Kabul-Kandahar] highway, and it hurts the other provinces as well."
Pakistani officials publicly deny that militants living in Baluchistan, the vast and volatile province in the southwest corner of the country, are helping to destabilize Afghanistan. But U.S. officials say senior Taliban leadership is living in Quetta, Baluchistan's capital, including the direct lieutenants of Taliban leader Mullah Omar.
U.S. military officials also say militants in Pakistan are providing much of the logistical help that the Afghan Taliban needs to sustain itself.
The Taliban, intelligence officers in Zabul say, have three supply lines into Zabul and, they believe, into the heart of southern Afghanistan. All three come from Pakistan.
The U.S. accusations are echoed by Afghan officials, who accuse Pakistan's military of turning a blind eye to the senior Taliban leadership.
"The leaders of the Taliban, all of them are living in Quetta, and they organize, they encourage and they lead the fight in southern Afghanistan," said Gen. Abdul Manan Farahi, Afghanistan's counterterrorism chief. "Pakistani trainers from Quetta cross over the border into Kandahar, Helmand, and Zabul."
Pakistani officials strongly deny that accusation. "We have a record of doing more than any other single country in the war against terrorism," says Pakistani information minister Qamar Zaman Kaira.
But what Pakistani officials do admit is that the border is impossible to seal.
"The border is a huge border, a porous border," says a senior Pakistani military official. "It is not possible to seal the border. The gaps remain."