The leader of the Islamic State of Iraq and al-Shams (ISIS,) Abu Bakr al-Baghdadi, was formerly held by the U.S. military at Camp Bucca in southern Iraq from 2005 to his release in 2009.
Why such a dangerous man was slated for release in 2009, or who made the decision is not known. The Telegraph offers that "one possible explanation is that he was one of thousands of suspected insurgents granted amnesty as the US began its draw down in Iraq."
In 2010, shortly after his release, al-Baghdadi was announced as a new al-Qaeda leader. When bin Laden was killed in 2011, Baghdadi pledged to revenge his death "with 100 terrorist attacks across Iraq" - but with al Qaeda leaders dropping like flies in Pakistan and Afghanistan, no one took him seriously.
But today, Baghdadi has instituted routine car bombings in Iraq and the death toll is now up over 1,000 per month.
This week, his forces seized control of government buildings in Mosul and are marching south to the capital, Baghdad. His forces have planted the black jihadi flag on towns from Fallujah to Ramadi, sites of previous U.S. victories.
So who is he?
The Telegraph reveals:
As with many of al-Qaeda's leaders, precise details are sketchy. His FBI rap sheet offers little beyond the fact that he is aged around 42, and was born as Ibrahim Ali al-Badri in the city of Samarrah...
"This guy was a Salafi (a follower of a fundamentalist brand of Islam), and Saddam's regime would have kept a close eye on him," said Dr Michael Knights, an Iraq expert at the Washington Institute for Near East Policy.
"He was also in Camp Bucca for several years, which suggests he was already considered a serious threat when he went in there."
That theory seems backed by US intelligence reports from 2005, which describe him as al-Qaeda's point man in Qaim, a fly-blown town in Iraq's western desert.
"Abu Duaa [an alias] was connected to the intimidation, torture and murder of local civilians in Qaim," says a Pentagon document. "He would kidnap individuals or entire families, accuse them, pronounce sentence and then publicly execute them."