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ERBIL, Iraq — Four years ago, when Lt. Gen. Michael Barbero was in charge of training Iraqi forces to hold their country together, he called them fearless and “absolutely reliable.”

But today, as Iraqi soldiers struggle against the ferocious militants of ISIS and the sting of humiliating defeat on the battlefield, the commander now says the United States was wrong to pull all its troops out of the country, with no residual force left behind to support the Iraqi army.

“We were wrong by not making a stronger case and insisting that we needed to stay in some numbers to continue to train, advise and develop this unfinished force,” Barbero told NBC News.

Barbero now works in the private sector and visits Iraq as a consultant. In 2010, he appeared much surer about the Iraqi forces he was training at the time. He said he was confident they were up to the task for maintaining security.

“I see a force that is capable, that is out on the streets every day providing security,” he said then. “They are reliable. They are absolutely reliable. They are fearless.”

But the rapid advance of ISIS into northern Iraq has eroded his confidence. When ISIS fighters advanced toward Mosul, Iraq’s second-largest city, two full Iraqi army divisions disintegrated as soldiers shed their uniforms and dropped their weapons.

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ISIS fighters soon posted videos of themselves driving American-made military vehicles and using American-made heavy weapons they captured from abandoned military posts.

“Until this past summer, until the advance of ISIS, I always believed that the forces holding Iraq together were stronger than the centrifugal forces pulling it apart,” Barbero said. “I’m not sure if I believe that anymore.”

He said that there were warning signs, such as corruption and infiltration from Shiite militias, but that the rapid collapse was nevertheless a shock.

Barbero said that the United States should pressure the government in Baghdad to reform its security forces and get rid of sectarian elements. The U.S. then would have to invest heavily, again, to train and equip Iraqi forces.

Active soldiers in the Iraqi army also told NBC News that they need more training. While they said they were willing to take on ISIS, they said they felt ill-prepared.

A 32-year-old sergeant from Baghdad, who requested anonymity for security reasons, said that the army was simply not prepared for battle and that it would take a long time to get rid of ISIS.

“We will be able to defeat ISIS, but not alone,” said the sergeant, who was trained by American forces in 2005 and served alongside U.S. advisers back then. “We need more training on how to handle things by ourselves.”

A fellow soldier, a 19-year-old private named Ahmed who is stationed in the province of Diyala, outside Baghdad, said that the Iraqi army simply can’t defeat ISIS alone.

More help could be on the way. Last week, President Barack Obama authorized 1,500 more troops to be sent to Iraq — doubling the presence of military advisors there. But it remains unclear whether that will be enough to shore up Iraqi forces against ISIS.

“We need U.S. and international troops to help us in the war against ISIS,” the young private said. “We are trying now to regain what we lost, but really we are moving so slow.”