The State Department's "Country Reports on Terrorism," a Congressionally mandated annual survey, cites the May 2011 killing of former Al-Qaeda leader Osama bin Laden as the "highlight" of the year's counterterrorism efforts.
Other top Al-Qaeda figures killed in 2011 include senior operational commander Atiya Abdul Rahman, who was killed in Pakistan in August, and Anwar al-Awlaki, chief of Al-Qaeda's external relations in the Arabian Peninsula, who was killed in Yemen in September.
Taken together, the report says, the losses incurred by Al-Qaeda "put the network on a path of decline that will be difficult to reverse."
Daniel Benjamin, the State Department's coordinator for counterterrorism, said Al-Qaeda's diminishing pull in the Muslim world was also illustrated during the popular uprisings of last year's Arab Spring.
"We saw millions of citizens throughout the Middle East advance peaceful, public demands for change without any reference to Al-Qaeda's incendiary world view. This upended the group's long-standing claim that change in this region would only come through violence," Benjamin said.
"These men and women have underscored in the most powerful fashion the lack of influence Al-Qaeda exerts over the central political issues in key Muslim-majority nations."
Numbers Down, Though Not Everywhere
According to U.S. data, setbacks to Al-Qaeda's core, coupled with improved counterterrorism capacity in multiple regions, contributed to the lowest worldwide tally of terrorist attacks since 2005.
The report says 10,283 terrorist incidents occurred in 70 countries last year, resulting in over 12,500 deaths. The numbers factor in a reduction in attacks of about 15 percent in both Afghanistan and Iraq.
Benjamin warned that despite the trend, the United States and the world "can't relax" in combating terrorism, and in particular, the threat of Sunni extremism, which continues to account for the majority of attacks.
The report found that Al-Qaeda's affiliates are strengthening in the Sahel and Arabian Peninsula, with the former gaining a foothold in Libya and Mali amid recent instability.
The downward trend also belies increases in terrorist attacks in Africa, due largely to Nigeria's Boko Haram militant group, as well as in the Western Hemisphere, where the report highlights the activity of the Revolutionary Armed Forces of Colombia (FARC).
Pakistan, Iran Highlighted
Extremist groups also continued to find "refuge" in Pakistan in 2011, contributing to an 8 percent increase in attacks in the country compared to 2010.
Benjamin cited "strong U.S. concern" about the activity of the Haqqani network, a Taliban-linked group based in Pakistan, and also called on Islamabad to "take more action" against the militant organization Lashkar-e Tayyiba.
Of the countries in focus on Washington's radar, however, Benjamin said Iran "remains the preeminent state sponsor of terrorism," engaging in its "most aggressive [terror] campaign since the 1990s."
"We are deeply concerned about Iran's activities on its own -- through the IRGC's [Islamic Revolutionary Guards Force] Quds Force -- and also together with Hizballah, as they pursue destabilizing activities around the globe," he said.
"We are firmly committed to working with partners and allies to counter and disrupt Iranian activities and to prevent Iran from sponsoring new acts of terror."
According to the report, Tehran increased its terrorist-related activities last year "likely in an effort to exploit the uncertain political conditions resulting from the Arab Spring, as well as in response to perceived increasing external pressure on Tehran."
The State Department cited the January 2011 bombing at Moscow's Domodedovo airport and the July 2011 right-wing attack in Norway as proof of terrorism's global scope.