In an audio tape released to jihadi forums the day after 9/11, al Qaeda chief Ayman al-Zawahiri laid out a strategy for future attacks. Zawahiri called for ‘a few of the brothers’ to commit small-scale, ‘sporadic blows’ in America. These types of attacks would “keep the U.S. in a constant state of heightened security,” he reasoned, which would in turn have a deleterious effect on the U.S. economy—a strategy al Qaeda has pursued for quite some time.
“We should bleed America economically by provoking it to continue in its massive expenditure on its security, for the weak point of America is its economy, which has already begun to stagger due to the military and security expenditure," he said.
An example of the strategy, which some refer to as ‘death by a thousand cuts,’ can be seen in the October 2010 ink cartridge bomb plot. In a special issue of Al Qaeda in the Arabian Peninsula’s online magazine, Inspire, the group boasts about the disparity between the cost of the plot to the terrorists and what it cost the United States. Via the Long War Journal (emphasis mine):
The cover of Inspire features "$4,200" in big, bold characters. AQAP's terrorists say this is the sum they spent on the operation. AQAP claims that for this small amount it has forced the West to spend billions of dollars in additional security, thereby making the operation effective even though no one was killed. […]
AQAP has dubbed the bomb plot "Operation Hemorrhage," with the idea being that al Qaeda and like-minded jihadists should launch numerous smaller attacks in order to force the West to spend exorbitant sums on additional layers of security.
In a separate piece, a cleric identified as Yahya Ibrahim lays out AQAP's strategy. "What has passed is the first of a multiphased operation," Ibrahim writes.
Although he stressed the importance of smaller attacks, Zawahiri said Muslims should not miss an opportunity for a major strike, even if they must wait years. So while the al Qaeda leader's message is nothing new, it's clear the group will continue to be a threat for decades to come.