PBS Frontline's "Yemen Under Siege," which aired on May 3, makes for powerful viewing. A first-hand look at the devastation that the U.S., Saudi Arabia, and other powers have visited on one of the poorest countries in the Middle East, the 35-minute documentary shows families struggling amid the rubble, children dying from mortar attacks, surgeons operating without anesthesia, and other such horrors.

But the most important revelation comes almost as an aside. Interviewing pro-Saudi fighters near the central Yemeni city of Taiz, journalist Safa Al Ahmad suddenly hears shouting. "What's wrong?" she asks. "Who are they? They don't want me to be here?"

A scene from PBS Frontline's "Yemen Under Siege."

A soldier explains that the people making a ruckus are Ansar al Sharia, i.e. fighters for shari'a. "And he just says quite casually, these are Al Qaeda in the Arabian Peninsula," Al Ahmad says later of the local Al Qaeda affiliate often referred to as AQAP. "And he referred to them by their local name, which is Ansar al Sharia. He revealed what is considered an open secret in the front lines, that they [AQAP] had been fighting with all the different factions, the [pro-Saudi] Yemeni factions and the [U.S.-Saudi] coalition against the Houthis."

"We don't accept you," the Al Qaeda members cry out. "On religious grounds, we do not accept you." A non-Al Qaeda fighter says dismissively, "They are ISIS." But a second corrects him: "No, they're not. They're worse than ISIS. We can't coexist with them."

But coexist they do, as the film makes clear. Yet another non-Al Qaeda fighter explains: "Islam does not allow for people to be overly strict. We must be moderate. But we have a group here who are strict."

"But you fight together at the front line?" Al Ahmad asks.