'Boots on the ground'
Despite repeated—and ill-advised, according to experts interview by CNBC—assurances that he would not be putting American troops into combat roles in Iraq, Obama has likely already deployed special forces to act against ISIS, Cordesman said. Additionally, the 1,100 military personnel that the president has publicly sent to Iraq to secure U.S. diplomatic facilities and staff are likely engaged in more offensive-minded roles than announced, he said.
But these numbers may not be nearly sufficient to accomplish Obama's stated goal of degrading and ultimately destroying ISIS, according to Brennan, who called the current deployment "a drop in the bucket" compared with what is needed.
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Now, the United States would need about 5,000 to 7,000 troops on the ground for an extended period of time to advise, train and assist the Iraqi and Kurdish forces sufficiently in the fight against ISIS, Brennan said.
"We need to get beyond the point of not thinking about putting American boots on the ground," he said. "In the long run that'll be insufficient to realizing the objectives [Obama] has established for himself."
Rubin, however, said he did not necessarily see the benefits of sending in more American ground troops, as that strategy often leads to "mission creep." Instead, he suggested that American military leaders look to the recapture of the Mosul Dam as a strategic model: American air power provided a "qualitative military edge" that allowed the regional forces to beat back ISIS.