As U.S. and NATO forces have evicted insurgents from a broad swath of southern Afghanistan, senior Taliban commanders have shifted toward a new battlefield strategy, one less focused on reclaiming lost territory and more on winning the next phase of the 11-year-old war.

U.S. military and intelligence officials believe that Taliban commanders, driven by a combination of desperation and savvy, have started assigning more of their suicide fighters to conduct audacious attacks against prominent targets across the country, including the U.S. Embassy and well-fortified NATO bases.

Insurgent leaders, they say, have redoubled a campaign to assassinate key Afghan government and security officials who are likely to play leadership roles in the country once foreign troops depart. And by happenstance or meticulous planning — U.S. military officials are not sure which — the Taliban has managed to kill numerous Western troops by joining the ranks of the Afghan army.

“The Taliban are fighting a political war while the United States and its allies are still fighting a tactical military war,” said Joshua Foust, a former U.S. intelligence analyst who has worked in Afghanistan and is now a fellow with the American Security Project. “We remain focused on terrain. They are focused on attacking the transition process and seizing the narrative of victory.”

The impact of the strategic shift, which has occurred gradually over the past year, has been profound. The high-profile assaults and assassinations have prompted new doubts among Afghans about the ability of their government and security forces to keep the insurgents at bay once NATO’s combat mission ends in 2014. The infiltration of the security forces led the top allied operational commander in Kabul on Monday to order extraordinary new restrictions on joint patrols and other missions, a move that strikes at the heart of the U.S. and NATO strategy to operate in closer partnership with Afghan soldiers.

U.S. officials said the new rules, which require high-level approval for partnered operations, also resulted from concern about possible attacks on American and NATO forces by Afghan troops enraged by reports of a controversial anti-Islam video produced in the United States. The officials said the operational commander will probably ease the restrictions in coming days if tensions over the video subside.

“Protecting the force for the moment protects the mission and the campaign later,” said a senior U.S. official in Kabul, who spoke on the condition of anonymity to discuss internal military decision-making.

The official said the pause in joint operations also will enable allied forces to implement new protection measures intended to defend against insider attacks resulting from Taliban infiltration, which account for about a quarter of all “green on blue” killings. Fifty-one foreign troops have been killed in insider attacks this year.

U.S. commanders have urged the Afghan Defense Ministry to take more aggressive actions to vet its troops for possible links to the Taliban. Until recently, the ministry resisted placing officers from the country’s intelligence service in army battalions to ferret out insurgents in the ranks.

It is not clear to U.S. military and intelligence officials whether the Taliban had long-standing plans to place sleeper agents within the Afghan security forces. If it did, some have wondered why the Taliban did not activate them sooner.

Regardless of how long the plans may have been in the works, U.S. officials think that Taliban leaders quickly sought to seize an opportunity to undermine confidence in the NATO mission. The infiltration was relatively easy, the officials said, because the Defense Ministry did little vetting, in part because of pressure from the American military to grow the Afghan army as quickly as possible.

The Taliban “made a decision at a senior level” to sneak into the security forces, said Alex Strick van Linschoten, an Afghanistan-based researcher who has co-written two books about the Taliban. “There wouldn’t be so many of them if this wasn’t officially sanctioned policy.”

Senior military officers said new force-protection measures will probably reduce the number of deaths from such attacks, but they acknowledged that insider assaults will remain a danger on the battlefield, just as roadside bombs are. “It’s not going to stop,” one U.S. general said. “But we’re not going to let these attacks get in the way of what we need to do.”

American commanders are fond of citing data that they think show major progress against the Taliban: From Jan. 1 to Aug. 31, enemy-initiated attacks dropped 5 percent compared with the same period last year; makeshift-bomb detonations have fallen 15 percent. Large parts of the south that once were teeming with insurgents now are relatively stable, although small bands of Taliban fighters continue to attack U.S. forces and reassert themselves, but with less vigor than a few years ago.

Even the southern city of Kandahar, once a key Taliban battleground, has been free of major attacks over the past few months, save for an audacious attempt to assassinate the provincial police chief with a 400-pound bomb packed into a fuel truck.

On Tuesday, Defense Secretary Leon E. Panetta called the insider attacks a “last-gasp effort” by the Taliban. Speaking during a visit to Beijing, he said the insurgents are trying “to create chaos because they have been unable to regain any of the territory that they have lost.”

U.S. commanders have offered a similar reaction to the Taliban’s other attempts to assert itself. Although the assassination campaign has claimed several key figures, including former president Burhanuddin Rabbani and police commander Daoud Daoud, Afghan government officials are still coming to work and performing their jobs. And, the commanders note, the high-profile assaults have usually been tactical failures that have resulted in most of the attackers being killed.

Even suicide bombings, horrific as they are, do not bring life to a halt in Kabul or other cities. On Tuesday morning, the Hezb-i-
Islami militant group, which has not been a major player in much of the recent violence, asserted responsibility for a suicide car bombing near the Kabul airport that killed 12 people, nine of them foreigners working for an air-charter company serving the U.S. Agency for International Development. A few hours later, traffic clogged nearby roads.

“The attacks give the perception that all of Afghanistan is like this, when that couldn’t be further from the truth,” the senior U.S. official in Kabul said.

One of the most significant Taliban attacks occurred Friday, when 15 well-trained insurgents clad in U.S. Army uniforms breached the perimeter of a large NATO air base in southwestern Afghanistan. They made it all the way to the flight line, where they killed two U.S. Marines, including a lieutenant colonel who commanded an aviation squadron, and destroyed six AV-8B Harrier jets and other equipment worth an estimated $200 million. The attack, which constituted the greatest single loss of allied materiel in the war, took out 7 percent of the Marine Corps’ overall fleet of combat Harriers.

“It’s a wake-up call,” the U.S. general said. “They’re saying, ‘Hey, we’re still here.’ ”

By demonstrating “the ability to strike at will against heavily guarded fortifications,” Foust said, “it shakes confidence, and it obscures the coalition’s metrics. Overall Taliban attacks may be down, but it doesn’t look that way to the Afghans — or the American people.”

To some military officials and independent analysts, the Taliban’s strategic shift suggests not just an adaptable adversary but greater involvement in the planning of attacks by elements of Pakistan’s intelligence service, which have long aided the Taliban.

Most of the recent high-profile attacks in Kabul, and many of the assassinations of senior Afghan leaders, have been conducted by fighters from the Pakistan-based Haqqani network. The officials and analysts see signs that the Haqqanis may be working more closely with the principal Taliban faction, led by the movement’s reclusive leader, Mohammad Omar.

The attack on the NATO base over the weekend “outstrips the ability of Mullah Omar to coordinate all of it,” said Jeffrey Dressler, an analyst at the Institute for the Study of War who tracks the Haqqani network. “Clearly there’s someone in the background pulling the puppet strings.”

So far, shifts in the Taliban’s strategy have not led the White House to halt troop reductions — all of the surge forces are scheduled to depart by the end of this month, leaving about 68,000 U.S. military personnel in Afghanistan — but the pattern of violence could affect how the winner of the presidential election chooses to adjust troop levels next year.

Regardless of how that decision plays out, the Taliban does not appear headed for defeat anytime soon. Large stretches of southern and eastern Afghanistan still remain in the grip of Omar’s faction or the Haqqanis, and incipient peace talks with the Afghan government do not appear to have gained traction.

“They know they’re not in a terrible place,” Dressler said. “They can lose every tactical battle, but they’re trumping it by winning the strategic narrative and the waiting game.”