Troops at Britain's main Afghan base, where Prince Harry was based, lowered their guard and left watchtowers empty before a spectacular Taliban assault that destroyed hundreds of millions of pounds' worth of American aircraft and equipment, a review has found.
Commanders at Camp Bastion had become preoccupied with halting deadly insider attacks by members of the Afghan forces and believed a frontal assault by the Taliban was unlikely.
As a result, they were unable to prevent 15 Taliban suicide attackers breaching the perimeter and causing the biggest single loss of American air power since the Vietnam War.
A critical United States Marine Corps review of the attack has found troops at the sprawling British base in Helmand province had "let their guard down", according to America's National Public Radio. The Taliban struck last September while Prince Harry was stationed there.
The attack was the second high–profile security breach at Bastion last year. Six months earlier, an Afghan worker drove a burning car at a reception party awaiting Leon Panetta, the US defence secretary.
Eight AV–8B Harrier jets belonging to the US marines were destroyed or badly damaged, and two marines were killed as British and American forces took more than five hours to kill the attackers. A US Congressional staff member familiar with the unpublished review told the radio network that allied military officers "considered this kind of an attack a low probability". They had instead been focused on "insider attacks".
The aide said security had since been boosted, and there was no indication anyone would be held accountable.
Bastion is also home to contingents from other nations, and troops from Tonga were reportedly guarding some of the perimeter at the time. The attackers had "got lucky" by cutting through an outer wire fence in a remote part of the 20 square mile base near the airstrip.
They used the rolling desert terrain and a dry stream bed to cover their night time approach, and had been disguised in American uniforms.
Some guard towers around the base had been left unattended.
Marines had thought the attackers were friendly forces until they opened fire with assault rifles and rocket–propelled grenades to destroy aircraft, hangars and fuel tanks.
Lt Col Chris Raible, a US marine squadron commander, was killed leading a counter attack, and Sgt Bradley Atwell was also killed. Several members of the RAF Regiment were wounded.
In the aftermath, a Taliban statement said they had attacked in retaliation for an American–made film insulting the Prophet Mohammed. Bastion had been chosen because of Prince Harry's presence, they added.
British and coalition investigations into the attack remain classified.
A spokesman for the Ministry of Defence said: "Security at Camp Bastion is constantly reviewed by [the coalition] and is appropriate to the currently assessed threat level.
"A thorough assessment of force protection was made by [the coalition], of which the UK is a member, following the incident."
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