In a major national security speech this spring, President Obama said again and again that the U.S. is at war with “Al Qaeda, the Taliban, and their associated forces.”

So who exactly are those associated forces? It’s a secret.

At a hearing in May, Sen. Carl Levin, D-Mich., asked the Defense Department to provide him with a current list of Al Qaeda affiliates.

The Pentagon responded – but Levin’s office told ProPublica they aren’t allowed to share it. Kathleen Long, a spokeswoman for Levin, would say only that the department’s “answer included the information requested.”

A Pentagon spokesman told ProPublica that revealing such a list could cause “serious damage to national security.”

“Because elements that might be considered ‘associated forces’ can build credibility by being listed as such by the United States, we have classified the list,” said the spokesman, Lt. Col. Jim Gregory. “We cannot afford to inflate these organizations that rely on violent extremist ideology to strengthen their ranks.” …

Jack Goldsmith, a professor at Harvard Law who served as a legal counsel during the Bush administration and has written on this question at length, told ProPublica that the Pentagon’s reasoning for keeping the affiliates secret seems weak. “If the organizations are ‘inflated’ enough to be targeted with military force, why cannot they be mentioned publicly?” Goldsmith said. He added that there is “a countervailing very important interest in the public knowing who the government is fighting against in its name.”

The law underpinning the U.S. war against Al Qaeda is known as the Authorization for Use of Military Force, or AUMF, and it was passed one week after the 9/11 attacks. It doesn’t actually include the words “associated forces,” though courts and Congress have endorsed the phrase. …