It will take a coalition of 50,000 troops on the ground to defeat the Islamic State, according to the former army chief of staff who spent more than four years serving in Iraq and who is credited, along with retired General David Petraeus, with being the architect of the successful 2007 troop surge there.

"Probably around 50,000," said Gen. Raymond T. Odierno during a panel discussion moderated by Fox News for the Foundation for Defense of Democracies. 

Odierno, who received the George P. Shultz award for distinguished service, emphasized the 50,000 would not all be U.S. troops, but the coalition would need to be U.S.- led.

While the general, who commanded all U.S. forces from 2008 to 2010, said he supports a unified country, he added the U.S. government needs to consider whether Iraq has already been divided into three sectors by the sectarian violence -- Shia, Sunni and Kurd. Odierno fingered the newly emboldened Iran as a primary agitator.

"Today, I think it's becoming harder and harder to have a unified Iraq,” he said. “And the reason is I believe the influence of Iran inside of Iraq is so great, they will never allow the Sunnis to participate in a meaningful way in the government. If that doesn't happen, you cannot have a unified Iraq."

Odierno, who argued for leaving 20,000 troops in Iraq but met resistance from several senior Obama administration officials as well as then Iraqi President Nouri al-Maliki, said the decision to pull out became a self-inflicted wound.

The withdrawal made it harder, if not impossible, for the U.S. government to independently assess what was happening on the ground, at a time when the alienation of the Sunni population fueled the rise of ISIS.

"We lost what we call our human intelligence network on the ground,” he said. “I mean we used to have a pretty significant human intelligence operation. So as we pulled out, our U.S. military, we lose it. So we have to depend on Iraqis, which they collect intelligence, but they do it a little bit differently than we do and they look for different things."

Speaking at the CIA Wednesday, President Obama touted the air campaign against ISIS, though Odierno said air power can only go so far, and working with the local Iraqis was the cornerstone of the surge.

When he was in Iraq, Odierno had first-hand knowledge of the ISIS leader Omar al-Baghdadi, who, at the time was a nondescript bomb maker with control over small Baghdad neighborhoods.

"We had captured him a couple of times, released him. He then fled to, I think, Syria. And then he shows (up) - and all of a sudden, I see him on TV making a pronouncement that he's the head of ISIS," Odierno recalled. "You have these individuals who've grown up now fighting the U.S. or whatever - an insurgency - and that becomes their life. And so they continue to grow and grow and grow and some of them become leaders of a movement, which is what he did.”

The retired general continued to sound the alarm about military cuts, saying the army has "lost capability" at a time when the likelihood of responding to threats on five continents is not hypothetical.

At the same time, the number of American troops dropped from over 100,000 to 50,000. In 2015, the White House sent 450 military advisers to train and assist Iraqi forces battling ISIS, with 5,000 troops.

Fox News' William Turner contributed to this report.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.