Governments combating terrorist groups can seem blind to these complications, especially to the consequences of weakening one group at the expense of another. When a state faces multiple terrorist adversaries—and there are almost always several different ones—encouraging splintering and factionalism, by killing top leaders for example, is not always the best option for reducing violence in the long run. Since at least the 1998 bombings of the American Embassies in East Africa, the U.S. has worked to undermine al-Qaeda, a campaign given new urgency and momentum after 2001. That goal has largely been accomplished if one thinks of al-Qaeda as the original organization Osama bin Laden founded around 1988. However, one affiliate (al-Qaeda in the Arabian Peninsula), one former affiliate now transformed into a rival (the Islamic State), and one potentially soon-to-be ex-affiliate (the Nusra Front) have emerged as formidable successors. And jihadist groups—some affiliated with al-Qaeda, some not—have proliferated in Africa and Asia.

What threatens American national security now, after more than a decade of U.S. operations against al-Qaeda, is primarily the rise of the Islamic State as a competitor to al-Qaeda and its allies of the moment. As the diagram below shows, the Islamic State's relationship to al-Qaeda and other groups has shifted over time.

The Islamic State's Relationships In Iraq

Kara Gordon (Data: Martha Crenshaw, Mapping Militant Organizations, Stanford University)

Under the leadership of its founder, Abu Musab al-Zarqawi (a Jordanian), the group joined bin Laden’s campaign shortly after the American invasion of Iraq in 2003. Known then as al-Qaeda in Iraq (AQI), it tried to dominate other Sunni groups fighting there but instead, through its ruthless tactics, provoked them to turn against it. At the same time, AQI also clashed with al-Qaeda central, which objected to its brutality (including videotaped beheadings) and acute sectarianism. Local disaffection, combined with the coalition military surge and perhaps Zarqawi’s death at the hands of the American military in 2006, caused the group to lose influence even though it rather grandly retitled itself the Islamic State of Iraq (ISI) at around the same time. This move was probably an effort to boost legitimacy and counter critics who accused AQI of being controlled by foreigners, but in hindsight the name change seems an important clue to the group’s intentions. By the time the Islamic State’s current leader, Abu Bakr al-Baghdadi, took over in 2010, ISI was a seriously weakened group that had lost most of its top leadership.

But after the withdrawal of foreign military forces from Iraq in 2011, ISI began to regain ground there in a much less pressured security environment. The outbreak of civil war in Syria in the same year provided ISI a new opportunity for expansion, which in turn led to the dissolution of its alliance with al-Qaeda central, as the following diagram of militant group relationships in Syria shows.