President Obama’s former ambassador to Iraq says in a new interview that his administration “did almost nothing” in response to intelligence warnings earlier this year that Islamic State radicals were gaining ground in Iraq and threatening the country’s stability.

“The administration not only was warned by everybody back in January, it actually announced that it was going to intensify support against ISIS with the Iraqi armed forces. And it did almost nothing,” says James Jeffrey, who served as U.S. ambassador to Iraq between 2010 and 2012, in "Frontline's" "The Rise of ISIS," which airs on PBS Tuesday night (check local listings) and is previewed here exclusively on Yahoo News.

Jeffrey is one of a number of ex-administration officials who appear in the film and sharply criticize the decisions of the president they once served. Former U.S. Ambassador to Syria Robert Ford and former Defense Secretary Leon Panetta both take issue with Obama’s refusal to arm moderate rebels in Syria who — it is now argued — could have acted as a counterweight to the Islamic State (also called ISIS or ISIL).

“I think we made the wrong decision in not providing assistance to the rebels,” Panetta bluntly says at one point.

The film, reported by correspondent Martin Smith, offers a richly detailed account of how the Iraqi government of Prime Minister Nouri al-Maliki alienated the country’s disenfranchised Sunni population, making reckless accusations of terrorism against Sunni leaders — including the country’s Vice Prime Minister Tariq al-Hashimi. Those allegations — flatly denied by al-Hashimi on camera — were based on the testimony of bodyguards who, it is strongly suggested, were tortured.

With little pressure or engagement from Washington, al-Maliki’s anti-Sunni agenda — driven by his “paranoia,” as one of Smith’s interlocutors says — paved the way for ISIS radicals to march through huge swaths of Iraqi territory this spring, seizing arsenals of U.S.-made weapons from a collapsing Iraqi army. This, of course, was the same army that the U.S. spent billions arming and training. In fact, terrorism expert Ken Katzman suggests in the film, they were a phantom led by do-nothing officers.

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“They were people who were — they were fat cats, I call them,” Katzman, a Congressional Research Service terrorism analyst, says in the film. “They were people who were earning good money to basically sit at a desk and smoke cigarettes and drink good liquor all day.”

In the end, Smith reports, it took only 800 ISIS militants, with the help of local Ba’athist military cadres, to secure Mosul, a city of 1.8 million people.

Ben Rhodes, President Obama’s chief national security spokesman, does his best to defend the administration record and at one point appears to blame Congress for holding up administration requests to step up arms supplies to the Iraqi Army.

“If you go back and you look at the record of what we were providing to the Iraqis, there was a steady increase, whether you're talking about Hellfire missiles, the Apaches,” says Rhodes. “They were held up by Congress; we sought the expedition of that delivery to the Iraqis.”

Perhaps the most striking exchange in the film comes as Smith presses Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, on whether the administration’s current policy of airstrikes at Islamic State targets will achieve Obama’s goal of “destroying” the Islamic State — without deploying U.S. ground troops.

"Are you an optimist at this point, that this really can work?" Smith asks him.

“No, I'm not an optimist,” says Dempsey in a less-than-confidence-building response. While the campaign’s strategy may be right, “every campaign's assumptions have to be revisited as the campaign evolves. Some of these assumptions are no doubt going to be challenged."

Smith’s film suggests this may end up meaning U.S. ground troops get involved — exactly what President Obama says he is determined to avoid but may ultimately have no choice but to accept.

