Here’s why the US training of foreign fighters hasn’t been successful

Instead of sending in America's sons and daughters, train locals to fight insurgents. That's the idea behind the billions of dollars the US has poured into training and equipping foreign armies and rebels.

In Iraq, the Pentagon trained and equipped the army and police. In Afghanistan, it backed the government forces. In Syria, the Defense Department spent $500 million to train and equip local rebels to fight the Islamic State.

But so far, all this investment hasn't yielded considerable success. In fact, earlier in September, it was reported that only four or five American-trained Syrian rebels were fighting the Islamic State. That's not a typo. Four or five.

Before that, a leader of the rebels was abducted along with his deputy.

US-trained and equipped Iraqi soldiers also fled their posts back in the summer when the Islamic State took over the city of Mosul. So why isn't the US plan to train and equip foreign fighters working?

Phillip Carter, a former army officer who's now with the Center for a New American Security, says these kinds of programs are extremely hard to get right. "Typically these programs work best when they are paired with a established government that just needs a little bit more support or a little extra financing for their programs," he says.

In countries where there is a weak government and a whole range of militias and fighters, like in Iraq and Syria, training and equipping isn't straight forward.

Back in 2005, Carter was an embedded adviser with the Iraqi police in the Diyala province. He says he had great difficulty creating both the expertise and the will among police officers to combat al-Qaeda. "We knew how to create an American police force or a Western police force," he explains, "we knew a lot less about how to take existing police and resources and create what the Iraqis wanted and needed."

Add to that, he says, other challenges such as language barriers and the inability to connect.

"Without excellent language ability and a human ability to connect and a sense of shared sacrifice, it's very difficult to partner with and train and advise foreign militaries," he says.

While there are parts of the US Special Operations community, he says, that excel at this but there are parts that still lack them. Carter goes on to say that while so far these programs haven't brought about great gains, he would caution against abandoning them.

In the case of Syria, for example, he thinks that the US has a broader interest in regional stability. It also wants to protect the interests of its allies such as Israel, Saudi Arabia and Jordan. But most importantly, he says, the US has a moral responsibility to the Syrian people.

"We cannot turn our back on them," he says.