In an interview on CBS "Sixty Minutes," former defense secretary Leon Panetta revealed that in the fall of 2012, the president's entire national security team was unanimous in wanting to arm the Free Syrian Army, which at the time, was the dominant rebel group fighting Bashar Assad.

But the president feared the weapons would fall into the wrong hands and declined to send weapons to the group.

How big a mistake was this? Scott Pelley's interview with Panetta is an eye opener:

It turns out President Obama was urged to intervene in Syria much earlier. In a new book, "Worthy Fights," former Defense Secretary Leon Panetta writes that, in a meeting in the fall of 2012 he, Secretary of State Hillary Clinton, the director of the CIA and the chairman of the Joint Chiefs all urged the President Obama to arm moderate Syrians who had started the revolution against the dictatorship to begin with. That might have left no room for ISIS to grow.

Leon Panetta: The real key was how could we develop a leadership group among the opposition that would be able to take control. And my view was, to have leverage to do that, we would have to provide the weapons and the training in order for them to really be willing to work with us in that effort.

Scott Pelley: But with virtually his entire national security team unanimous on this, that's not the decision the president made.

Leon Panetta: I think the president's concern, and I understand it, was that he had a fear that if we started providing weapons, we wouldn't know where those weapons would wind up. My view was, "You have to begin somewhere."

Scott Pelley: In retrospect now, was not arming the rebels at that time a mistake?

Leon Panetta: I think that would've helped. And I think in part, we pay the price for not doing that in what we see happening with ISIS.

Now ISIS has forced Mr. Obama to reverse himself. The new policy depends on local forces to win on the ground, but many of the available partners are dubious. Syrian rebels fight each other. And the Pentagon figures only about half the Iraqi army is reliable.

The Free Syrian Army was originally a collection of individual soldiers and small units who had defected from the Syrian armed forces. Most were draftees. They had very little organization until larger units and general officers of the Syrian army began to join them. When that happened, the rebels formed a military council with the FSA at its head and they began receiving arms from Gulf states like Saudi Arabia and the UAE.

There was a point in late summer, early fall of 2012, where an infusion of weapons by the west might have been a boon to the FSA, who had just begun to prove themselves on the battlefield. The Islamists groups - including ISIS - were scattered and not very effective (with the exception of the Al-Nusra front which maintained an uneasy distance from the FSA). The FSA was as secular a group as we were going to find - even more secular than the civilian opposition council that was made of a majority of Muslim Brotherhood members.

That is the situation that confronted the US when the president made his decision not to arm the FSA. A lot more went into that decision than simply the worry that the weapons we sent would fall into the hands of extremists. But the president declined to take the risk and history will be the ultimate judge of his failure.