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.
All Taliban Supply Lines to Zabul Go Through Pakistan
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."
Local Pakistani officials say the border is only a formality. The same tribes live on both sides, the same language is spoken on both sides, and the border drawn by departing European colonial rulers 115 years ago does not have any historic significance for the ethnic Pashtuns who dominate the area. Families are allowed to cross without passports.
Asked if Taliban cross the border from Pakistan into Afghanistan, Mulvi Agha Muhammad, a member of the Pakistani parliament from a district just across the border from Zabul, said, "Of course they do, even as we speak … all Afghan men have beards and differentiating between a Talib and a non-Talib is extremely difficult. They are all citizens of Afghanistan and they all have beards, so who are you supposed to stop from crossing over?"
The United States has never sent enough resources to the area to secure the border. In Zabul, the border became "one of the lowest priorities, given the resources that I have," admits Cannata.
Afghan Border Police: Short-Staffed and Ill-Equipped
Before a few months ago, neither he nor any of his predecessors in Zabul even knew how to call the lone border police post on the Zabul-Baluchistan border. He now has a mobile phone number for the border police chief, but cannot visit the area without a huge amount of reinforcements -- not because he fears an attack but because he simply does not have enough intelligence to know what kind of resistance might be waiting for him when he arrives.
"Coming in, I was just asking myself, 'What's my border picture?' I couldn't find it," he said in an interview in his office in the main base in Zabul, just outside the provincial capital, Qalat.
On the wall a huge map of the province details precisely where U.S. troops are located, where their coalition partners the Romanian troops are located, and the location for every casualty his unit has suffered. But in the area near the border, there are almost no push-pins, no color-coordinated stickers.
"Unfortunately, the border is not well-covered at all," Cannata says, "nowhere near what I would like as a military professional."
Just as the United States has not had enough resources, so too have the Afghan border police been woefully short-staffed.
"In my opinion, our border police don't have enough equipment to stop the Taliban," says Gen. Farahi, the Afghan counterterror chief. "The number of police is not enough. The area is a very big area, and we can't control all of the fighters coming across the border."
That was evident to Cannata during an incident in May when the border police station in Zabul was attacked.
Eventually, after realizing he did not have any contact numbers, he reached the police chief and asked where the station was in relation to the attackers. The chief was a bit flummoxed: he did not have a single map or grid in his station to help explain the geography of the area.
"I was on the floor, trying to compare my maps to the village names he was giving me," Cannata recalled, shaking his head while telling the story.
He has reason to worry. The supply lines from Pakistan, he says, come into Zabul and then cross Highway 1, the road that links Kabul to Kandahar, the largest city in southern Afghanistan. At those crossings, militants concentrate their attacks using roadside bombs, which have become the greatest worry for U.S. troops and for commercial traffic that use the highway.
Securing the Afghan Border: What Next?
The U.S. is now sending more than 20,000 troops to southern Afghanistan. Approximately half are Marines, who will deploy to Helmand, which also shares a border with Pakistan, and the western province of Farah. The other half are mostly Stryker brigade soldiers who will deploy to Kandahar and Zabul.
That will help, Cannata says, especially if they are able to fan out across enough districts to successfully protect the Afghan population from the Taliban, who roam freely near the border. If that happens, the local population will provide the U.S. with more intelligence and both American and Afghan soldiers will be better able to defend the porous border.
Publicly, the Pakistani government has challenged the U.S. surge, doubting whether it will work and voicing concerns that it could push militants from Afghanistan into Pakistan.
Often, Pakistani Taliban fighters are led by Afghan commanders.
But when challenged, Pakistani officials acknowledge that the Afghan militants who are fighting in Pakistan are in the tribal areas, not in Baluchistan. And they acknowledge that even since one of the largest battles of the war began in Helmand earlier this month, very few militants have crossed the border from Afghanistan into Pakistan.
"Insignificant numbers have crossed the border from Helmand," says the senior Pakistani military official.