This was the enemy, served up in the latest chart from the intelligence agencies: 15 Qaeda suspects in Yemen with Western ties. The mug shots and brief biographies resembled a high school yearbook layout. Several were Americans. Two were teenagers, including a girl who looked even younger than her 17 years.

President Obama, overseeing the regular Tuesday counterterrorism meeting of two dozen security officials in the White House Situation Room, took a moment to study the faces. It was Jan. 19, 2010, the end of a first year in office punctuated by terrorist plots and culminating in a brush with catastrophe over Detroit on Christmas Day, a reminder that a successful attack could derail his presidency. Yet he faced adversaries without uniforms, often indistinguishable from the civilians around them.

“How old are these people?” he asked, according to two officials present. “If they are starting to use children,” he said of Al Qaeda, “we are moving into a whole different phase.”

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