British authorities Sunday chargedfour men in Birmingham with plotting a terrorist bombingcampaign in the United Kingdom, accusing two of the alleged cell members oftravelling to Pakistan for "training in terrorism including bomb making,weapons and poison making" at some point after Christmas Day 2010. The DailyTelegraph reportedlast week that British authorities suspect that al-Qaeda operatives in Pakistanmay have directed the plot.

Underliningthe seriousness of the plot, three of their number were charged with "beingconcerned in the purchase of components and chemicals for a home made explosivedevice,"  and "construction of a homemade explosive device for terrorist acts."

Thealleged Birmingham plot highlights the fact that despite mounting pressure from dronestrikes on al-Qaeda Central in Pakistan, Westernmilitants receiving terrorist training in the tribal areas of Pakistan arestill a significant homeland security threat to Western countries.

This was the conclusion of an in-depth study Iauthored for the New America Foundation in July, which included a comprehensivesurvey of all the serious terrorist plots against the West since 2004.

The study found that progress against al-Qaedain Pakistan had not yet been reflected in the metric that most counts, that ofreduced plots against the West originating with or involving the FederallyAdministered Tribal Areas, or FATA.

Last year, there were four serious Islamistterrorist plots against the West with training or operational links toestablished groups in Pakistan, the most in any year since al-Qaedaconsolidated its safe-haven in the FATA soon after the 9/11 attacks and theAmerican invasion of Afghanistan. The alleged Birmingham plot is the secondsuch plot thwarted in 2011. In April of this year, German police broke up an alleged plot centered on Dusseldorf by German residents trained and directed by al-Qaedain Pakistan.

Since 2010 there has only been one serious plotdirected against the West from Yemen. While al-Qaeda in the Arabian Peninsula (AQAP)has garnered significant attention from Western officials and analysts, thepresence of several terrorist groups in Pakistan with a track record oftargeting the West, arguably makes the Afghanistan-Pakistan border region stillthe most dangerous terrorist safe haven in the world. For example, FaisalShahzad who attempted to bomb Times Square on May 1, 2010 was trained by thePakistani Taliban earlier that year, and was directed to launch his failedattack by the group.

While drone strikes in FATA have undoubtedlydamaged al-Qaeda, the organization has to some degree adapted by decentralizingits operations and training militants indoors inside small mountain shacks,according to the testimony of Western recruits who recently trained with militant groups in the region. As outlined inthe New America Foundation study, al-Qaeda has also promoted new recruits intosenior positions, including Western recruits with a keen understanding ofWestern vulnerabilities.

For example American-Saudi AdnanShukrijumah allegedly helped al-Qaeda orchestrate the September 2009 plot against NewYork's Subway involving Afghan immigrant Najibullah Zazi, and is still believedat large in the tribal areas. So is Abdullrahmen Hilal Hussain, an Austrian-bornmilitant of Syrian descent, who  hasallegedly helped organize bomb-making instruction for Western recruits, according tocourt documents. 

Some operatives who have been killed orarrested will, however, be very difficult to replace. Time will tell if thedeath of al-Qaeda leader Osama bin Laden, the recent killing of several key alQaeda operatives in drone strikes the tribal areas of Pakistan, including Ilyas Kashmiri and Atiyah Abd al-Rahman, aswell as the arrest of senior al-Qaeda operative Younis al-Mauretani in Quetta in August, will reduce the number ofplots being directed  against the Westeach year with links to terrorist groups in Pakistan.

Hundreds of Western militants are currentlytraining or operating in Pakistan, according to an official report published by the U.K. Home Office in July.Western-counter-terrorism officials say recruits are still streaming into thetribal areas of Pakistan from the West. While most travel there to fight inAfghanistan, their transit through the area provides al-Qaeda withopportunities to launch terrorist attacks in their home countries.

Furthermore a Pakistani military operation toremove the presence of pro-al-Qaeda militants from North Waziristan -- theepicenter of plots against the West in recent years - appears as remote asever. While groups like the Tehrik-i-Taliban Pakistan (TTP) and the Haqqani Networkremain ensconced in these areas, they will likely continue to protect andharbor al-Qaeda.

In a survey of the 32 serious plots against theWest between January 2004 and early July 2011 the study found that 44 percentof these plots had direct operational ties to terrorist groups in Pakistan,throwing into sharp relief the danger posed by the FATA terrorist safe-haven.The proportion of serious plots in which cell members trained with terroristgroups in Pakistan was higher still - 53 percent of all such plots against theWest. By way of contrast, only 6 percent of these plots had operational ortraining ties to terrorists in Yemen, and only 3 percent to Iraq. In only 38percent of serious plots was there no overseas training.

The study categorized "serious" plots as allthose in which weapon components had been obtained without the assistance ofundercover law enforcement agents which had the capacity to kill at least ten.

The full New America Foundation study isavailable here

Paul Cruickshank, an investigative reporterspecializing in al Qaeda, is an alumni Fellow at the NYU Center on Law &Security and a terrorism analyst for CNN. The views expressed in this articleare entirely his own.