A cluster of militant attacks over the past week is a reminder of how the once-singular threat of al Qaeda has changed since the killing of Osama bin Laden, morphing or splintering into smaller, largely autonomous Islamist factions that in some cases are now overshadowing the parent group.
Between 2010 and 2013, the number of al Qaeda and al Qaeda-related groups rose 58 percent and the number of "Salafi jihadists" - violent proponents of an extreme form of Islam - more than doubled, according to a report by the RAND Corp think tank.
Daniel Benjamin, former U.S. State Department counterterrorism coordinator under President Barack Obama, said he was "considerably more optimistic 18 months ago than ... now" about the threat posed by al Qaeda-related groups.
Few examples are more vivid than the fall of northern Iraq, which has raised the prospect of the country's disintegration as a unified state.
Sunni insurgents known as the Islamic State of Iraq and the Levant, or ISIL, seized the northern city of Mosul on Tuesday, and then overran an area further south on Wednesday, capturing the city of Tikrit and threatening Iraq's capital, Baghdad.
The militants are exploiting deep resentment among Iraq's Sunni minority, which lost power when the 2003 U.S.-led invasion toppled Saddam Hussein. Since the U.S. withdrawal in 2011, the Sunni population has become increasingly alienated from Prime Minister Nouri al-Maliki's Shii'ite-dominated government and his U.S.-trained military.
This has helped fuel the stunning resurgence of ISIL. The group seeks to create a caliphate based on medieval Sunni Islamic principles across Iraq and neighboring Syria, where it has become one of the fiercest rebel forces in the civil war to oust President Bashar al-Assad.
ISIL underscores the complexity of the new galaxy of militant groups. Earlier this year, it split from the core al Qaeda organization completely, after a dispute between ISIL's leader and bin Laden's successor, Ayman al-Zawahiri.
"WE ARE TALKING ABOUT YEARS"
Even if Iraq can survive the onslaught, there is no saying how long it might take to restore order. "This is a very protracted war against terror," said an adviser to Maliki. "We are not talking about months. We are talking about years."
It has taken years for the situation to reach its current low point. After the 2003 Iraq invasion, the disgruntled Sunni population initially served as the base for a bloody insurgency against the U.S. military and emerging Shi'ite majority rule.
That revolt appeared to have been quelled by the time U.S. troops left in December 2011. But Iraqi Sunni grievances simmered, fanned by what they saw as Maliki's sectarian rule and failure to build an inclusive government and army.
The future members of ISIL, then calling themselves the Islamic State of Iraq, were ready when the uprising in Syria started in 2011 and moved in to take advantage of the chaos. Bolstered by their success on the battlefield, they renamed themselves the Islamic State of Iraq and the Levant.
With ISIL's lightning advance in Iraq in recent days, the army has seen thousands of soldiers desert their posts in the north. And in Baghdad, fears of a sectarian bloodbath have grown.
Benjamin, now at Dartmouth University, said that groups like ISIL and rival Jabhat al-Nusrah in Syria, while serious regional problems, do not pose the same direct threat to the United States and its allies that bin Laden's al Qaeda did.
"We shouldn't lose sight of that," he said. "I don't think it's an existential threat by any means."
TENSIONS HIGH IN PAKISTAN