The Afghan war is returning to the place it began: the violent eastern borderlands with Pakistan, where the Taliban and al-Qaeda slipped out of American reach a decade ago and have organized their insurgency ever since.

In southern Afghanistan, the United States has succeeded over the past year in prying the Taliban’s grip from parts of Kandahar and Helmand provinces. But U.S. military commanders recognize they have far to go in the country’s east, where insurgents fight from the cover of craggy mountains, drive truckloads of weapons through illegal dirt-road border crossings, and flee across the frontier into Pakistan to elude capture.

The intense U.S. focus on the south has meant that there are about 38,500 troops in that region, compared with 31,000 in eastern Afghanistan. But those in the east have borne a disproportionately high share of casualties in recent months, and some territory held by the Afghan government has fallen back into Taliban hands after U.S. troops pulled out of their small outposts.

In eastern Afghanistan, “we really haven’t focused our energy and efforts,” said U.S. Army Lt. Gen. David Rodriguez, NATO’s second-ranking commander in Afghanistan. “Because you can’t do it everywhere at the same time.”

By concentrating more on the east, U.S. military officials hope to confront the cross-border flow of Taliban and Haqqani network fighters who operate from Pakistan’s poorly governed tribal districts. The higher priority would mean more intelligence capabilities, such as surveillance drones, as well as more Afghan soldiers for the region. But commanders are faced with the problem of trying to intensify a fight with fewer American troops, as President Obama begins withdrawing forces next month.

With less combat power, commanders must balance between keeping troop-strength high in the south to hold their gains and shifting more to the problems in the east. In the past six months, 64 U.S. troops have died in the east, compared with 67 in the south, despite the fact that there are 7,500 more troops in the south. Some U.S. planners have made the case for making the east the war’s top priority as soon as this summer, but Rodriguez said that is unlikely to happen.

“It’s the last place we will be fighting,” a senior U.S. military official said, speaking on the condition that he not be identified by name. “And the Afghans will be fighting there in perpetuity. It’s a bad neighborhood.”

The problems in the east start with Pakistan, whose tribal border districts have long provided refuge for Afghan insurgents. Fighters for the Taliban, as well as al-Qaeda and the Pakistani group Lashkar-i-Taiba, can move from Pakistan into places such as Konar province, which has cultivated a toxic mix of fighters in remote mountain valleys. U.S. military commanders recognized last year that they were likely never going to have enough troops to pursue a strategy built around protecting the Afghan population.

Starting in 2006, the United States kept an 800-member battalion in the Pech Valley — enough to secure the ground to pave over a dirt trail into the valley, but not enough to extend the reach of the Afghan government, oversee large construction projects or defeat the Taliban in one of the most violent parts of the country. So a new strategy emerged: The troops withdrew from the valley this spring and began to use mobile units based in Konar and outside Jalalabad, a 45-minute helicopter ride away, to conduct large sweeps against insurgent strongholds every few months. So far, these raids have taken place in the Pech and Konar river valleys, the two major arteries in the province.

Farther south along the border, the problems do not relent. The provinces of Khost, Paktika and Paktia form the traditional stronghold of the Haqqani network, the roughly 3,000 men fighting for guerrilla commander Jalaluddin Haqqani and his sons, who run their war from just over the border in Pakistan’s North Waziristan tribal region.

Five years ago, when the U.S. military had one brigade working in all of eastern Afghanistan, Col. Christopher Toner worked in this area as a battalion commander. He recalled a shortage of aircraft, an almost nonexistent police force and regular ambushes by dozens of insurgents. “We didn’t have the resources,” Toner said. “The enemy definitely enjoyed a freedom of movement.”

After returning as the commander of a 5,000-man brigade in Khost this year, one of seven brigades now in the east, Toner said that “it’s absolutely night and day between then and now” because the Afghan government and security forces have developed and U.S. troops can apply more pressure on Haqqani fighters. But Toner’s soldiers are still stretched thin enough that they have trouble covering all the routes into Afghanistan or patrolling all the insurgent strongholds in the area. “We’re still a little short” of troops, Toner said.

“The insurgents can win just by hanging on,” said one U.S. official in Khost. “And I think we’re all aware of that.”

Commanders here recognize they are in a bare-knuckled fight. In neighboring Paktika, a province the size of Massachusetts, American troops have fired more than 12,000 artillery rounds in the past nine months, including some across the border into Pakistan. The 3,500-man brigade there has killed nearly 400 people, and it has been attacked 146 times by bombs and 547 times by mortar and rocket rounds. Just four members of the Taliban have chosen to stop fighting and join the government’s reintegration program.

“It’s a highly kinetic area,” said Col. Sean M. Jenkins, who commands the U.S. Army brigade in Paktika and said his troops were arrayed against insurgents who are proving unusually skilled.

Pakistan’s military has done little to confront some factions, particularly those crossing the border to fight in Afghanistan. Although U.S. troops cooperate with their counterparts in Pakistan’s army and Frontier Corps on some operations, the relationship tends to be one of frustration and suspicion. During his recent trip to Afghanistan, Defense Secretary Robert M. Gates likened the U.S.-Pakistan relationship to a “troubled marriage.”

After the killing of Osama bin Laden, a humiliation for Pakistan’s military and intelligence service, Pakistan canceled some of its “border flag” coordination meetings with U.S. soldiers, ratcheting up the tension. Lt. Col. Jesse Pearson, a battalion commander in Khost, across from North Waziristan, said he has met one of the two Pakistani army brigade commanders across the border from him, but that single meeting is as far as the partnership has gone.

U.S. military officials expect they might add troops to parts of eastern Afghanistan in coming years, even as they draw down elsewhere. But for now, commanders on the ground are not expecting much additional help.

“I’ve got about four months to win the war,” said Pearson, the battalion commander. “It’s well known that by the July-August time frame, we’re going to have the most combat forces that we’re ever going to have.”

“I have got to push as hard as I possibly can now,” he said.