The Taliban fighters who blew up a half-dozen U.S. Marine fighter jets on a sprawling NATO base last fall were able to walk easily onto the encampment because patrols of the perimeter had been scaled back and watchtowers left unmanned, according to senior military officials.

After the attack, which resulted in the deaths of two Marines and the largest loss of allied materiel in the 11-year-long Afghan war, the top U.S. commander on the base did not order a formal investigation into the security lapses or sanction any personnel responsible for guarding the facility, the officials said.

In the days following the raid, some U.S. and NATO military leaders insisted that the Taliban got lucky by choosing to breach where they did. But several officials with direct knowledge of the assault said in recent interviews that staffing decisions by U.S. and British commanders weakened the base’s defenses, making it easier for the insurgents to reconnoiter the compound and enter without resistance. Once inside, 15 insurgents used grenades to destroy almost an entire squadron of Marine AV-8B Harrier jets, a loss estimated by military officials at about $200 million.

The new account of the attack illustrates the challenge of withdrawing troops from Afghanistan in the coming months. Unlike Iraq, from which U.S. forces departed largely peacefully, Afghanistan remains perilous. U.S. intelligence officials say Taliban operatives are on the lookout for opportunities to strike if U.S. and NATO commanders reduce security as they send home thousands of troops.

An unmanned tower

The attack occurred on Camp Bastion, a British-run NATO air base in Helmand province that adjoins Camp Leatherneck, a vast U.S. Marine facility that serves as the NATO headquarters for southwestern Afghanistan. Because Leatherneck does not have a runway, the Marines use Bastion as their principal air hub. Several hundred Marines live and work on the British side, and dozens of U.S. helicopters and fixed-wing aircraft park there.

The British are responsible for guarding Bastion, which is ringed by a chain-link fence and triple coils of razor wire. There are numerous tall watchtowers on the perimeter, which allow sentries to scan the horizon for potential attackers. Instead of placing their own soldiers in the towers, British commanders handed the job to troops from the Pacific island of Tonga, which has sent 55 soldiers to Afghanistan.

On the night of the attack — Sept. 14 — the tower closest to the Taliban point of entry was unmanned, according to four U.S. military officials who were at Camp Leatherneck at the time or received briefings about the incident later. Those officials and other military officers interviewed spoke on the condition of anonymity because they were not authorized to discuss the attack or security measures at Camp Bastion, key elements of which remain classified.

Several watchtowers at Bastion were manned at the time, but the Tongan soldiers in those posts could not observe the area around the empty tower, the officials said. “There was dead space,” one official said.

A senior U.S. officer with direct knowledge of security on the bases said it was common for Bastion’s watchtowers to be unmanned. Long-standing U.S. military security protocols permit towers to be unoccupied only if other posts can observe the unmanned tower’s entire area of responsibility.

“Security at Camp Bastion is constantly reviewed . . . and is appropriate to the currently assessed threat level,” the British Ministry of Defense said in a statement responding to questions from The Washington Post.

U.S. Marines responsible for protecting Camp Leatherneck and Camp Bastion have not relied on watchtowers alone to protect the facilities. Since U.S. troops established Leatherneck in 2009, teams of Marines have patrolled a 230-square-mile swath of desert around the bases in an attempt to identify Taliban activity and persuade residents in makeshift villages there to report on suspicious activity.

In December 2011, 325 Marines were assigned to patrol the area, according to the senior U.S. officer with knowledge of security measures. In the month before the attack, however, the number was cut back to about 100, the officer said.

“We reduced the force to patrol [the area around the base] to the point where it was an unacceptable risk,” the officer said.

The top U.S. commander at Leatherneck at the time, Marine Maj. Gen. Charles Gurganus, who approved of the reduction, said force levels for perimeter patrols were based on U.S. and NATO assessments of possible threats. At the time, other military officials said, Marine intelligence analysts did not issue any specific warnings of a frontal assault by the Taliban.

“You can’t defend everywhere every day,” Gurganus said in response to a question about the attack. “You base your security on the threat you’ve got.” He said the Taliban caught “a lucky break.”

“When you’re fighting a war, the enemy gets a vote,” he said.

Other military officials said security incidents on the bases in the preceding months should have prompted increased vigilance. In March 2012, an Afghan interpreter working for British forces at Camp Bastion drove a flaming vehicle toward a U.S. military aircraft carrying then­Defense Secretary Leon E. Panetta. The vehicle crashed into a ditch and missed Panetta’s plane.

Marine officers also had been concerned about the performance of the Tongan troops, who were sometimes found asleep at their posts.

The senior officer with knowledge of security matters said some officers raised questions about the reduction of Marines to patrol the perimeter with Gurganus and were told that it was an “acceptable risk.”

In the months leading up to the September attack, Gurganus and his predecessor had to withdraw about 10,000 U.S. troops from Helmand, reducing total force levels to about 7,000. That cutback, the senior officer said, increased pressure to trim the patrol.

Despite the troop reduction, several officers stationed at Leatherneck at the time said many Marines with idle time could have been assigned to guard duty. Instead, some of them took online college classes and others worked out in the gym twice a day. And two weeks after the attack, a Texas hold ’em poker tournament was held at one of the recreation centers at Leatherneck.

“There was plenty of manpower to assign to base security,” one of the officers said. “It just wasn’t a priority.”

Detailed reconnaissance

Military officials believe the reduction in patrols allowed the insurgents to conduct detailed reconnaissance of the airfield before the attack, identifying the times during which guard towers were unoccupied and sketching maps of where planes were parked. The insurgents even publicized the maps after the attack.

On the night of the attack, Taliban insurgents dressed in what officials believe were stolen U.S. military uniforms slipped onto the base and destroyed the aircraft. Two Marines, Lt. Col. Christopher Raible, 40, and Sgt. Bradley Atwell, 27, were killed.

In the wake of the attack, the top U.S. and NATO commander in Afghanistan, Marine Gen. John R. Allen, assigned his deputy, Lt. Gen. Adrian Bradshaw of the British army, to investigate the incident.

Gurganus ordered a colonel on his staff to conduct a review of security procedures on the bases. Gurganus did not order a formal Marines Corps judge advocate general investigation into the attack. He said he could not because Bastion is a NATO facility.

Other senior officers, however, contended that a formal U.S. investigation was warranted because two Marines were killed and so many aircraft were destroyed.

When the House Armed Service Committee asked to see the initial Marine security review earlier this year, senior officers on the Pentagon’s Joint Staff deemed it insufficient for release and ordered the Marines to conduct a fuller review, military officials said. But that examination still fell short of an official investigation.

No U.S. or British military personnel have been reprimanded as a result of the attack. The Marine Corps does not plan to release its review. NATO also intends to keep its investigation confidential, in part to avoid embarrassing the British for leaving towers unmanned, according to officers briefed on the findings.

“We’ve corrected the deficiencies in security, but we don’t want to put a stick in the eye of our closest ally,” said a senior U.S. official who served in Afghanistan at the time.

Instead, the Marine Corps has sought to focus attention on the heroic response of Marine aviators during the attack. Mechanics, supply clerks and pilots grabbed rifles and ran onto the flight line to fight the insurgents, who had split into three five-man teams. Raible, a Harrier squadron commander, charged into the combat zone armed with only a handgun.

Had Raible and his fellow Marines not responded so quickly, senior officers said they believe, the human toll and property damage would have been far worse. “Their actions prevented an even greater loss,” one of the officers said. “But they never should have been put in that position in the first place.”